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Full text of "Worlds in the making; the evolution of the universe"

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M C M V I I I 


c 6X 


Copyright, 1908, by HARPER & BROTHERS. 

All rights reserved. 
Published March, 1908. 



Destruction caused by volcanism and by earthquakes. 
Different kinds of volcanoes. Vesuvius. Products of erup- 
tion. Volcanic activity diminishing. Structure of vol- 
canoes. Geographical distribution of volcanoes. Tempera- 
ture in the interior of the earth. Significance of water 
for volcanism. Composition of the earth's interior. Geo- 
graphical distribution of earthquakes. Fissures in the 
earth's crust. Groups of earthquakes. Waves in the sea 
and in the air accompanying earthquakes. Their connec- 
tion with volcanism. Systems of fissures. Seismograms. 



Manifold character of the worlds. The earth probably at 
first a ball of gases. Formation of the earth crust and its 
rapid cooling. Balance between heat received and heat 
lost' by radiation. Life already existing on the earth for 
a milliard of years. The waste of solar heat. Temperature 
and habitability of the planets. Heat-preserving influence 
of the atmosphere. Significance of carbon dioxide in the 
atmosphere Warm and cold geological ages. Fluctua- 
tions in the percentage of carbon dioxide of the air. Com- 
bustion, decay, and growth. Atmospheric oxygen. Vege- 
table life more ancient than animal life. The atmospheres 
of planets. Chances of an improvement in the climate. 


Stability of the solar system. Losses and possible gains 
of heat by the sun. Theses of Mayer and of Helmholtz. 
Temperatures of the white, yellow, and reddish stars, and 
of the sun. Sun-spots and sun faculse. Prominences. 


Spectra of the parts of the sun. Temperature of the sun. 
The interior of the sun. Its composition according to the 
mechanical theory of heat. The losses of heat by the sun 
probably covered by the enormous solar energy. 


Newton's law of gravitation. Kepler's observation of 
comets' tails. The thesis of Euler. Proof of Maxwell. 
The radiation pressure. Electric charges and condensa- 
tion. Comets' tails and radiation pressure. Constituents 
and properties of comets' tails. Weight of the solar corona. 
Loss and gain of matter by the sun. Nature of meteor- 
ites. Electric charge of the sun. Electrons drawn into the 
sun. Magnetic properties of the sun and appearance of the 
corona. Constituents of the meteors. Nebulae and their 
heat and light. 



The supply of dust from the sun rather insignificant. 
Polarization of the light of the sky. The upper clouds. 
Different kinds of aurorse. Their connection with the 
corona of the sun. Polar lights and sun-spots Periodicity 
of polar lights. Polar lights and magnetic disturbances. 
Velocity of solar dust. -Fixation of atmospheric nitrogen. 
The Zodiacal Light. 


The extinction of the sun. Collision between two celestial 
bodies. The new star in Perseus. Formation of nebulae. 
The appearance of nebulae. The nebulae catch wandering 
meteors and comets. The ring nebula in Lyra. Variable 
stars. Eta in Argus. Mira Ceti. Lyra and Algol stars. 
Evolution of the stars. 


The energy of the universe. The entropy of the universe. 
The entropy increases in the suns, but decreases in the 
nebulae. Temperature and constitution of the nebulae. 
Schuster's calculations of the condition of a celestial body 
consisting of gases. Action of the loss of heat on nebulae 


and on suns. Development of a rotating nebula into a 
planetary system. The hypothesis of Kant-Laplace. Ob- 
jections to it. The views of Chamberlin and Moulton. 
The radiation pressure balances the effect of Newtonian 
gravitation. The emission of gases from the nebulae bal- 
ances the waste of heat characteristic to the solar systems. 


UNIVERSE ... ............ 212 

Stability of the species. Theory of mutation. Sponta- 
neous generation. Bathybius. Panspermia. The stand- 
points of Richter, Ferdinand Cohn, and Lord Kelvin. The 
radiation pressure enables spores to escape. The effect of 
strong sunlight and of cold on the germinating power. 
Transport of spores through the atmosphere into universal 
space and through it to other planets. General conclusions. 


The temperatures are stated in degrees centigrade ( C.), either on 
the Celsius scale, on which the freezing-point of water is O, or on 
the absolute scale, whose zero lies 273 degrees below the freezing- 
point of water, at 273 C. The equivalent temperatures on the 
Fahrenheit scale (freezing-point of water 32 F.) are added in 
brackets ( F.). 

1 metre (m.) =10 decimetres (dm.) =100 centimetres (cm.) =1000 

millimetres (mm.) =3.28 ft.; 1 kilometre (km.) =1000 metres (rn.) ^ v 
= 1.6 miles ; 1 mile =0.62 kilometres (km.). 

Light travels in yacuo at the rate of 300,000 km. (nearly 200,000 
miles) per second. 




MODERATE ACTIVITY ........ .... 2 

2. ERUPTION OF VESUVIUS IN 1882 ...... .... 4 

3. ERUPTION OF VESUVIUS IN 1872 .......... 6 


ASHES ........... . ..... 8 

5. BLOCK LAVA ON MAUNA LOA .......... 10 


TERTIARY AGE .............. 11 


"NECK" ................ 12 







EARTHQUAKE OF 1906 ........... 25 


FLOODED TREES ............. 27 









AUGUST 31, 1898 35 














27. THE GREAT SUN-SPOT OF OCTOBER 9, 1903 ...... 80 












ENLARGEMENT 1 : 70 109 



BERING STRAIT, 1879 124 



SPITZBERGEN, 1883 126 

V viii 






ON NOVEMBER 15 AND 16, 1905 138 


15 AND 16, 1905 139 


45. SPECTRUM OF NOVA AURIGA, 1892 . 154 


















WHEN, more than six years ago, I was writing my 
Treatise of Cosmic Physics, I found myself confronted 
with great difficulties. The views then held would not 
explain many phenomena, and they failed in particular 
in cosmogonic problems. The radiation pressure of 
light, which had not, so far, been heeded, seemed to % 
give me the key to the elucidation of many obscure 
problems, and I made a large use of this force in dealing 
with those phenomena in my treatise. 

The explanations which I tentatively offered could, of 
course, not claim to stand in all their detail; yet the 
scientific world received them with unusual interest and 
benevolence. Thus encouraged, I tried to solve more of 
the numerous important problems, and in the present 
volume I have added some further sections to the com- 
plex of explanatory arguments concerning the evolution 
of the Universe. The foundation to these explanations 
was laid in a memoir which I presented to the Academy 
of Sciences at Stockholm in 1900. The memoir was soon 
afterwards printed in the Physikalische Zeitschrift, and 
the subject was further developed in my Treatise of 
Cosmic Physics. 

It will be objected, and not without justification, that 
scientific theses should first be discussed and approved of 
in competent circles before they are placed before the 
public. It cannot be denied that, if this condition were 



to be fulfilled, most of the suggestions on cosmogony that 
have been published would never have been sent to the 
compositors; nor do I deny that the labor spent upon their 
publication might have been employed for some better 
purpose. But several years have elapsed since my first 
attempts in this direction were communicated to scien- 
tists. My suggestions have met with a favorable recep- 
tion, and I have, during these years, had ample op- 
portunity carefully to re-examine and to amend my 
explanations. I therefore feel justified in submitting 
my views to a larger circle of readers. 

The problem of the evolution of the Universe has al- 
ways excited the profound interest of thinking men. And 
it will, without doubt, remain the most eminent among 
all the questions which do not have any direct, practical 
bearing. Different ages have arrived at different solu- 
tions to this great problem. Each of these solutions re- 
flected the stand-point of the natural philosophers of its 
time. Let me hope that the considerations which I offer 
will be worthy of the grand progress in physics and chem- 
istry that has marked the close of the nineteenth and 
the opening of the twentieth century. 

Before the indestructibility of energy was understood, 
cosmogony merely dealt with the question how matter 
could have been arranged in such a manner as to give 
rise to the actual worlds. The most remarkable con- 
ception of this kind we find in Herschel's suggestion of 
the evolution of stellar nebulae, and in the thesis of La- 
place concerning the formation of the solar system out 
of the universal nebula. Observations more and more 
tend to confirm Herschel's view. The thesis of Laplace, 
for a long time eulogized as the flower of cosmogonic 
speculations, has more and more had to be modified. If 
we attempt, with Kant, to conceive how wonderfully 



organized stellar systems could originate from absolute 
chaos, we shall have to admit that we are attacking a 
problem which is insoluble in that shape. There is a 
contradiction in those very attempts to explain the origin 
of the Universe in its totality, as Stallo 1 emphasizes: 
" The only question to which a series of phenomena gives 
legitimate rise relates to their filiation and interdepend- 
ence." I have, therefore, only endeavored to show how 
nebulae may originate from suns and suns from nebulae; 
and I assume that this change has always been proceed- 
ing as it is now. 

The recognition of the indestructibility of energy 
seemed to accentuate the difficulties of the cosmogonic 
problems. The theses of Mayer and of Helmholtz, on the 
manner in which the Sun replenishes its losses of heat, 
have had to be abandoned. My explanation is based 
upon chemical reactions in the interior of the Sun in 
accordance with the second law of thermodynamics. 
The theory of the "degradation" of energy appeared to 
introduce a still greater difficulty. That theory seems 
to lead to the inevitable conclusion that the Universe 
is tending towards the state which Clausius has desig- 
nated as " Wdrme Tod" (heat death), when all the energy 
of the Universe will uniformly be distributed through 
space in the shape of movements of the smallest particles. 
That would imply an absolutely inconceivable end of the 
development of the Universe. The way out of this diffi- 
culty which I propose comes to this: the energy is " de- 
graded" in bodies which are in the solar state, and the 
energy is "elevated," raised to a higher level, in bodies 
which are in the nebular state. 

Finally, I wish to touch upon one cosmogonical ques- 

1 Stallo : Concepts and Theories of Modern Physics. London, 1900, 
p. 276. 



tion which has recently become more actual than it used 
to be. Some kind of "spontaneous generation," origina- 
tion of life from inorganic matter, had been acquiesced in. 
But just as the dreams of a spontaneous generation of 
energy i.e., of a perpetuum mobile have been dispelled 
by the negative results of all experiments in that direction, 
just in the same way we shall have to give up the idea of 
a spontaneous generation of life after all the repeated 
disappointments in this field of investigation. As 
Helmholtz 1 says, in his popular lecture on the growth 
of the planetary system (1871) : " It seems to me a per- 
fectly just scientific procedure, if we, after the failure of 
all our attempts to produce organisms from lifeless matter, 
put the question, whether life has had a beginning at 
all, or whether it is not as old as matter, and whether 
seeds have not been carried from one planet to another 
and have developed everywhere where they have fallen 
on a fertile soil." 

This hypothesis is called the hypothesis of panspermia, 
which I have modified by combining it with the thesis 
of the radiation pressure. 

My guiding principle in this exposition of cosmogonic 
problems has been the conviction that the Universe in 
its essence has always been what it is now. Matter, en- 
ergy, and lifs have only varied as to shape and position 
in space. 


STOCKHOLM, December, 1907. 

1 Helmholtz, Popular^ Wissenschaftliche Vortrage. Braunschweig, 
1876, vol. iii., p. 101. 


The Interior of the Earth 

THE disasters which have recently befallen the flour- 
ishing settlements near Vesuvius and in California have 
once more directed the attention of mankind to the terrific 
forces which manifest themselves by volcanic eruptions 
and earthquakes. 

The losses of life which have been caused in these two 
last instances are, however, insignificant by comparison 
with those which various previous catastrophes of this 
kind have produced. The most violent volcanic eruption 
of modern times is no doubt that of August 26 and 27, 
1883, by which two-thirds of the island of Krakatoa, 33 
square kilometres (13 square miles) in area, situated in 
the East Indian Archipelago, were blown into the air. 
Although this island was itself uninhabited, some 40,000 
people perished on that occasion, chiefly by the ocean 
wave which followed the eruption and which caused 
disastrous inundations in the district. Still more terrible 
was the destruction wrought by the Calabrian earth- 
quake of February and March, 1783, which consisted of 



several earthquake waves. The large town of Messina 
was destroyed on February 5th, and the number of 
people killed by this event has been estimated at 100,000. 
The same region, especially Calabria, has, moreover, fre- 
quently been visited by disastrous earthquakes again 
in 1905 and 1907. Another catastrophe upon which 
history dwells, owing to the loss of life (not less than 
90,000), was the destruction of the capital of Portugal 
on November 1, 1755. Two-thirds of the human lives 
which this earthquake claimed were destroyed by a wave 
5 m. in height rushing in from the sea. 

Vesuvius is undoubtedly the best studied of all vol- 
canoes. During the splendor of Rome this mountain was 
quite peaceful known as an extinct volcanic cone so far 
as history could be traced back. On the extraordinari- 
ly fertile soil about it had arisen big colonies of such 
wealth that the district was called Great Greece (Grsecia 

Fig. 1. Vesuvius, as seen from the Island of Nisida, in 
moderate activity 

Magna). Then came, in the year 79 A. D., the devastating 
eruption which destroyed, among others, the towns of 
Herculaneum and Pompeii. The volumes of gas, rushing 
forth with extreme violence from the interior of the 
earth, pushed aside a large part of the volcanic cone 
whose remnant is now called Monte Somma, and the 
falling masses of ashes, mixed with streams of lava, 



built up the new Vesuvius. This mountain has repeat- 
edly changed its appearance during later eruptions, 
and was provided with a new cone of ashes in the 
year 1906. The outbreak of the year 79 was succeeded 
by new eruptions in the years 203, 472, 512, 685, 993, 
1036, 1139, 1500, 1631, and 1660, at quite irregular 
intervals. Since that time Vesuvius has been in al- 
most uninterrupted activity, mostly, however, of a harm- 
less kind, so that only the cloud of smoke over its crater 
indicated that the internal glow was not yet extinguished. 
Very violent eruptions took place in the years 1794, 
1822, 1872, and 1906. 

Other volcanoes behave quite differently from these 
violent volcanoes, and do hardly any noteworthy dam- 
age. Among these is the crater-island of Stromboli, situ- 
ated between Sicily and Calabria. This volcano has been 
in continuous activity for thousands of years. Its erup- 
tions succeed one another at intervals ranging from one 
minute to twenty minutes, and its fire serves the sailors 
as a natural light-house. The force of this volcano is, of 
course, unequal at different periods. In the summer of 
1906 it is said to have been in unusually violent activity. 
Very quiet, as a rule, are the eruptions of the great vol- 
canoes on Hawaii. 

Foremost among the substances which are ejected from 
volcanoes is water vapor. The cloud floating above the 
crater is, for this reason, the surest criterion of the activity 
of the volcano. Violent eruptions drive the masses of 
steam up into the air to heights of 8 km. (5 miles), as the 
illustrations (Figs. 1 to 4) will show. 

The height of the cloud may be judged from the height 
of Vesuvius, 1300 metres (nearly 4300 ft.) above sea- 
level. The illustration on page 4 (Fig. 2) is a repro- 
duction of a drawing by Poulett Scrope, representing the 


Vesuvius eruption of the year 1822. There seems to 
have been no wind on this day; the masses of steam 
formed a cloud of a regular shape which reminds us of a 
pine-tree. According to the description of Plinius, the 
cloud noticed at the eruption of Vesuvius in the year 79 

Fig. 2. Eruption of Vesuvius in 1882. (After a 
contemporaneous drawing by Poulett Scrope) 

must nave been of the same kind. When the air is not so 
calm the cloud assumes a more irregular shape (Fig. 3). 
Clouds which rise to such elevations as we have spoken 
of are distinguished by strong electric charges. The 



vivid flashes of lightning which shoot out of the black 
clouds add to the terror of the awful spectacle. 

The rain which pours down from this cloud is often 
mixed with ashes and is as black as ink. The ashes have 
a color which varies between light -gray, yellow -gray, 
brown, and almost black, and they consist of minute 
spherules of lava ejected by the force of the gases and 
rapidly congealed by contact with the air. Larger drops 
of lava harden to volcanic sand the so-called "lapilli" 
(that is, little stones), or to "bombs," which are often 
furrowed by the resistance offered by the air, and turn 
pear-shaped. These solid products, as a rule, cause the 
greatest damage due to volcanic eruptions. In the year 
1906 the weight of these falling masses (Fig. 4) crushed 
in the roofs of houses. A layer of ashes 7 m. (23 ft.) in 
thickness buried Pompeii under a protective crust which 
had covered it up to days of modern excavations. The 
fine ashes and the muddy rain clung like a mould of 
plaster to the dead bodies. The mud hardened after- 
wards into a kind of cement, and as the decomposition 
products of the dead bodies were washed away, the 
moulds have provided us with faithful casts of the ob- 
jects that had once been embedded in them. When the 
ashes fall into the sea, a layer of volcanic tuffa is formed 
in a similar manner, which enshrines the animals of the 
sea and algse. Of this kind is the soil of the Campagna 
Felice, near Naples. Larger lumps of solid stones with 
innumerable bubbles of gases float as pumice-stone on 
the sea, and are gradually ground down into volcanic 
sand by the action of the waves. The floating pum- 
ice-stone has sometimes become dangerous or, at any 
rate, an obstacle to shipping, through its large masses; that 
was, at least, the case with the Krakatoa eruption of 



Among the gases which are ejected in addition to 
water vapor, carbonic acid should be mentioned in the 
first instance; also vapors of sulphur and sulphuretted 
hydrogen, hydrochloric acid, and chloride of ammonium, 
as well as the chlorides of iron and copper, boric acid, 
and other substances. A large portion of these bodies 
is precipitated on the edges of the volcano, owing to 

Fig. 3. Eruption of Vesuvius in 1872. (After a photograph. ) 

the sudden cooling of the gases. The more volatile con- 
stituents, such as carbonic acid, sulphuretted hydrogen, 
and hydrochloric acid, may spread over large areas, and 
destroy all living beings by their heat and poison. It 
was these gases, for example, which caused the awful 
devastation at St. Pierre, where 30,000 human lives were 
destroyed on May 8, 1902, by the eruption of Mont Pelee. 
The ejection of hydrogen gas, which, on emerging from 



the lava, is burned to water by the oxygen of the air, has 
been observed in the crater of Kilauea. 

The ashes of the volcanoes are sometimes carried to 
vast distances by the air currents e. g., from the west- 
ern coast of South America to the Antilles; from Iceland 
to Norway and Sweden; from Vesuvius (1906) to Hoi- 
stein. Best known in this respect is the eruption of 
the Krakatoa, which drove the fine ashes up to an ele-- 
vation of 30 km. (18 miles). The finest particles of 
these ashes were slowly carried by the winds to all parts 
of the earth, where they caused, during the following 
two years, the magnificent sunrises and sunsets which 
were spoken of as "the red glows." This glow was also 
observed in Europe after the eruption of Mont Pelee. 
The dust of Krakatoa further supplied the material for 
the so-called "luminous clouds of the night/' which were 
seen in the years 1883 to 1892 floating at an elevation 
of about 80 km. (50 miles), and hence illuminated by the 
light of the sun long after sunset. 

The crater of Kilauea, on the high volcano of Mauna Loa, 
in Hawaii this volcano is about of the same height as 
Mont Blanc has excited special interest. The crater forms 
a large lake of lava having an area of about 12 sq. km. (near- 
ly 5 sq. miles), which, however, varies considerably with 
time. The lava boiling at red glow is constantly emitting 
masses of gas under slight explosions, spurting out fiery 
fountains to a height of 20 in. (65 ft.) into the air. Here 
and there lava flows out from crevices in the wall of the 
crater down the slope of the mountain, until the surface 
of the lake of lava has descended below these cracks. 
As a rule, this lava is of a thin fluid consistency, and it 
spreads, therefore, rather uniformly over large areas. 
Of a similar kind are also the floods of lava which are 
sometimes poured over thousands of square kilometres 


Fig. 4. Photograph of Vesuvius, 1906. Chiefly clouds of ashes 

on Iceland. The so-called Laid eruption of the year 1783 
was of a specially grand nature. Though occurring in 
an uninhabited district, it did a great amount of dam- 
age. In the more ancient geological periods, especially 
in the Tertiary age, similar sheets of lava of vast ex- 
tensions have been spread over England and Scotland 
(more than 100,000 sq. km., roughly, 40,000 sq. miles); 
over Deccan, in India, 400,000 sq. kins. (150,000 sq. 
miles), up to heights of 2000 m. (6500 ft.); and over 
Wyoming, Yellowstone Park, Nevada, Utah, Oregon, 
and other districts of the United States, as well as over 
British Columbia. 

In other cases the slowly ejected lava is charged 
with large volumes of gases, which escape when the lava 



congeals and burst it up into rough, unequal blocks, 
forming the so-called block lava (Fig. 5). The streams 
of lava can likewise produce terrible devastation when 
they descend into inhabited districts; on account of their 
slow motion, they rarely cause loss of life, however. 

Where the volcanic activity gradually lessens or ceases, 
we can still trace it by the exhalations of gas and the 
springs of warm water which we find in many districts 
where, during the Tertiary age, powerful volcanoes were 
ejecting their streams of lava. To this class belong the 
famous geysers of Iceland, of Yellowstone Park (Fig. 
6), and of New Zealand; also the hot springs of Bo- 
hemia, so highly valued therapeutically (e. #., the Karls- 
bad Sprudel); the Fumaroli of Italy, Greece, and other 
countries, exhaling water vapor; the Mofettse, with their 
exhalations of carbonic acid (of frequent occurrence in 
the district of the Eifel and on both sides of the middle 
Rhine, in the Dogs Grotto near Naples, and in the Valley 
of Death in Java) ; the Solf atara, exhaling vapors of sul- 
phur sulphuretted hydrogen and sulphur dioxide (they 
are found near Naples on the Phlegrsean Fields and in 
Greece) ; as well as many of the so-called mud volcanoes, 
which eject mud, salt water, and gases (as a rule, car- 
bonic acid and hydrocarbons) for example, the mud 
volcanoes near Parma and Modena, in Italy, and those 
near Kronstadt, in Transylvania. 

The extinct volcanoes, of which some, like the Acon- 
cagua, 6970 m. (22,870 ft.), in South America, and the 
Kilimanjaro, in Africa, 6010 m. (19,750 ft.), rank among 
the highest mountains, are exposed to a rapid destruc- 
tion by the rain, because they consist largely of loose 
materials volcanic ashes with interposed layers of lava. 
Where these lava streams expand gradually, they pro- 
tect the ground underneath from erosion by water, and 



Fig. 5. Block lava on Mauna Loa 

in this way proper cuts .are formed on the edges of the 
lava streams, passing through the old volcano and through 
the sedimentary strata at deeper levels. 

The old volcano of Monte Venda, near Padua, affords 
an interesting example of this type. We can observe 
there how the sedimentary limestone has been changed 
by the lava, which was flowing over it, into marble to a 
depth of about 1 m. (3 ft.) Sometimes the limestone 
which is lying over the lava has also undergone the same 
transformation, which would indicate that lava has not 
only been flowing above the edge of the crater, but has 

also forced itself out on the 'sides through the fissures 



< CO 


between two layers of limestone. Massive subterranean 
lava streams of this kind are found in the so-called lak- 
kolithes of Utah and in the Caucasus. There the supe- 
rior layers have been forced upward by the lava pressing 
from below; the lava froze, however, before it reached 
the surface of the earth, where it might have formed a 
volcano. Quite- a number of granites, the so-called 
batholithes, chiefly occurring in Norway, Scotland, and 
Java, are of similar origin. Occasionally it is only the 
core of congealed lava that has remained of the whole 

volcano. These cores, 
which originally filled 
the pipe of the crater, 
are frequent in Scot- 
land and in North 
America, where they 
are designated "necks" 
(Fig. 7). 

The so-called canons 
of the Colorado Pla- 
teau, with their almost 
vertical walls, are the 
results of the erosive 

action of rivers. A drawing by Button shows a wall of 
this kind more than 800 m. (2600 ft.) in height, through 
four fissures of which lava streams have forced their way 
up to the surface (Fig. 8). Over one of these fissures a 
small cone of volcanic ashes is still visible, while the cones 
which probably overtopped the three other fissures have 
been washed away, so that the veins end in small "necks." 
Evidently a very fluid lava strong percentages of mag- 
nesia and of oxide of iron render the lava more fluid 
than an admixture of silicic acid, and the fluidity is 

further increased by the presence of water has been 


Fig. 7. Mato Tepee in Wyoming, 
U. S. A. Typical volcanic " Neck " 


forced into the fissures which were already present, and 
has reached the surface of the earth before it froze. The 



Fig. 8. Clefts filled with lava and volcanic cone of ashes, Torowheap 
Canon, Plateau of Colorado. Diagram. 

driving force behind them must have been pretty strong; 
else the lava streams could not have attained the neces- 
sary velocity of flow. 

When the Krakatoa was blown into the air in 1883 
half of the volcano remained behind. This half clearly 
shows the section of the cone of ashes, which has been 
but very slightly affected by the destructive action of 
the water. We find there in the central part the light- 
colored stopper of lava in the volcano pipe, and issuing 
from it more light-colored beds of lava, between which 
darker strata of ashes can be seen. 

The distribution of volcanoes over the surface of the 
earth is marked by striking regularities. Almost all the 
volcanoes are situated near the shores of the sea. A few 
are found in the interior of East Africa; but they are, 
at any rate, near the Great Lakes of the equatorial 


regions. The few volcanoes which are supposed to be 
situated in Central Asia must be regarded as doubtful. 
We miss, however, volcanoes on some sea-coasts, as in 
Australia and along the long coast-lines of the Northern 
Arctic Ocean to the north of Asia, Europe, and America. 
Volcanoes occur only where great cracks occur in the 
crust of the earth along the sea-coast. Where such fis- 
sures are found, but where the sea or large inland lake 
basins are not near as, for instance, in the Austrian 
Alps we do not meet with any volcanoes ; such districts 
are, however, renowned for their earthquakes. 

Since ancient ages the belief has been entertained that 
the molten masses of the interior of the earth find an 
outlet through the volcanoes.. Attempts have been made 
to estimate the depth of the hearths of volcanoes, but 
very different values have been deduced. Thus, the 
hearth under the volcano of Monte Nuovo, which was 
thrown up in the year 1538 on the Phlegrsean Fields, near 
Naples, has been credited with depths varying from 1.3 km. 
to 60 km. (1 mile to 40 miles) ; for the Krakatoa, estimates 
of more than 50 km. (30 miles) have been made. All these 
calculations are rather aimless; for the volcanoes are 
probably situated on folds of the earth-crust, through 
which the fluid mass (the magma) rushes forth in wedges 
from the interior of the earth, and it will presumably be 
very "difficult to say where the hearth of magma ends 
and where the volcanic pipe commences. The Kilauea 
gives the visitor the impression that he is standing over 
an opening in the crust of the earth, through which the 
molten mass rushes forth directly from the interior of the 
earth. (Fig. 9.) 

As regards the earth-crust, we know from observations 
in bore-holes made in different parts of the world that 
the temperature increases rather rapidly with the depth, 



on an average by about thirty degrees Cent, per kilometre 
(about 1.6 F. per 100 feet). It must be remarked, how- 
ever, that the depth of our deepest bore-holes hardly 
exceeds 2 km. (Paruchowitz, in Silesia, 2003 m., or 
6570 ft.; Schladebach, near Merseburg, Prussian Saxony, 
1720 m.). If the temperature should go on increasing 
at the rate of 30 degrees Cent, for each further kilometre, 
the temperature at a depth of 40 kilometres should attain 
degrees at which all the common rocks would melt. But 
the melting-point certainly rises at the same time as the 
pressure. The importance of this circumstance was, 

Fig. 9. The Kilauea Crater on Hawaii 

however, much exaggerated when it was believed that for 
this reason the interior of the earth might possibly be 
solid. Tammann has shown by direct experiments that 
the temperature of fusion only rises up to a certain press- 



ure, and that it begins to decrease again on a further 
increase of pressure. The depths indicated above are 
therefore not quite correct. If we assume, however, that 
other kinds of rock behave like diabase the melting- 
point of which, according to the determinations of Barus, 
rises by 1 Cent, for each 40 atmospheres of pressure 
corresponding to a depth of 155 m. we should conclude 
that the solid crust of the earth could not have a greater 
thickness than 50 or 60 km. (40 miles). At greater 
depths we should therefore penetrate into the fused 
mass. On account of its smaller density the silicic acid 
will be concentrated in the upper strata of the molten 
mass, while the basic portions of the magma, which are 
richer in iron oxide, will collect in the lower strata, owing 
to their greater density. 

This magma we have to picture to ourselves as an ex- 
tremely viscid liquid resembling asphalt. The experi- 
ments of Day and Allen show that rods, supported at 
their ends, of 30 x 2 x 1 mm. of different minerals, like the 
feldspars microcline and albite, could retain their shape for 
three hours without curving noticeably, although their 
temperature was about a hundred degrees above their 
melting-point, and although they appeared completely 
fused, or, more correctly, completely vitrified-^wihen taken 
out of the furnace. These molten silicates behave very 
differently from other liquids like water and mercury, with 
which we are more accustomed to deal. 

The motion and diffusion in the magma, and especially 
in the very viscous and sluggish acid portions of the upper 
strata, will therefore be exceedingly small, and the mag- 
ma will behave almost like a solid body, like the minerals 
of the experiments of Day and Allen. The magmas of 
volcanoes like Etna, Vesuvius, and Pantellaria may, 
therefore, have quite different compositions, as we should 



conclude from their lavas without our being forced to 
believe, with Stubel, that these three hearths of vol- 
canoes are completely separated, though not far removed 
from one another. In the lava of Vesuvius a tempera- 
ture of 1000 or 1100 degrees has been found at the lower 
extremity of the stream. From the occurrence in the 
lava of certain crystals like leucite and olivin, which we 
have reason to assume must have been formed before 
the lava left the crater, it has been concluded that the 
lava temperature cannot have been higher than 1400 
degrees before it left the volcanic pipe. 

It would, however, be erroneous to deduce from the 
temperature of the lava of Vesuvius that the hearth of 
the volcano must be situated at a depth of approximate- 
ly 50 kilometres. Most likely its depth is much smaller, 
perhaps not even 10 kilometres. For there, as every- 
where where volcanoes occur, the crust of the earth is 
strongly furrowed/and the magma will just at the spots 
where we find volcanoes come much nearer to the sur- 
face of the earth than elsewhere. 

The importance of water for the formation of vol- 
canoes probably lies in the fact that, in the neighbor- 
hood of cracks under the bottom of the sea, the water 
penetrates down to considerable depths. When the 
water reaches a stratum of a temperature of 365 degrees 
--the so-called critical temperature of water it can no 
longer remain in the liquid state. That would not prevent, 
however, its penetrating still farther into the depths, in 
spite of its gaseous condition. As soon as the vapor comes 
in contact with magma, it will eagerly be absorbed by the 
magma. The reason is that water of a temperature of 
more than 300 degrees is a stronger acid than silicic acid ; 
the latter is therefore expelled by it from its compounds, 
the silicates, which form the main constituents of the 



magma. The higher the temperature, the greater the 
power of the magma to absorb water. Owing to this 
absorption the magma swells and becomes at the same 
time more fluid. The magma is therefore pressed out by 
the action of a pressure which is analogous to the osmotic 
pressure by virtue of which water penetrates through a 
membrane into a solution of sugar or salt. This press- 
ure may become equivalent to thousands of atmos- 
pheres, and this very pressure would raise the magma 
up the volcanic pipe even to a height of 6000 in. 
(20,000 feet) above the sea-level. As the magma is as- 
cending in the volcanic pipe it is slowly cooled, and its 
capacity for binding water diminishes with falling tem- 
perature. The water will hence escape under violent 
ebullition, tearing drops and larger lumps of lava with 
it, which fall down again as ashes or pumice-stone. After 
the lava has flown out of the crater and is slowly cooling, 
it continues to give off water, breaking up under the for- 
mation of block lava (see Fig. 5). If, on the other hand, 
the lava in the crater of the volcano is comparatively at 
rest, as in Kilauea, the water will escape more slowly; 
owing to the long - continued contact of the surface 
layer of lava with the air, little water will remain in it, 
the water being, so to say, removed by aeration, and the 
lava streams will therefore, when congealing, form more 
smooth surfaces. 

In some cases volcanoes have been proved (Stiibel and 
Branco) not to be in connection with any fractures in 
the crust of the earth. That holds, for instance, for sev- 
eral volcanoes of the early Tertiary age in Swabia. We 
may imagine that the pressure produced by the swelling 
of the magma became so powerful as to be able to break 
through the earth-crust at thinner spots, even in the 
absence of previous fissures. 



If, in our consideration, we follow the magma farther 
into the depths, we shall not find any reason for assum- 
ing that the temperature will not rise farther towards the 
interior of the earth. At depths of 300 or 400 km. (250 
miles) the temperature must finally attain degrees such 
that no substance will be able to exist in any other state 
than the gaseous. Within this layer the interior of the 
earth must, therefore, be gaseous. From our knowledge 
of the behavior of gases at high temperatures and press- 
ures, we may safely conclude that the gases in the cen- 
tral portions of the earth will behave almost like an ex- 
tremely viscid magma. In certain respects they may 
probably be compared to solid bodies; their compressi- 
bility, in particular, will be very small. 

We might think that we could not possibly learn any- 
thing concerning the condition of those strata. Earth- 
quakes have, however, supplied us with a little informa- 
tion. Such gaseous masses must fill by far the greatest 
part of the earth, and they must have a very high specific 
gravity; for the average density of the earth is 5.52, and 
the outer strata, the ocean and the masses of the surface 
which are known to us, have smaller densities. The or- 
dinary rocks possess a density ranging from 2.5 to 3. 
It must, therefore, be assumed that the materials of the 
innermost portions of the earth must be metallic, and 
Wiechert, in particular, has advocated this view. Iron 
will presumably form the chief constituent of this gas 
of the central earth. Spectrum analysis teaches us that 
iron is a very important constituent of the sun. We 
know, further, that the metallic portions of the meteo- 
rites consist essentially of iron; and finally terrestrial 
magnetism indicates that there must be large masses of 
iron in the interior of the earth. We have also reason 
to believe that the native iron occurring in nature e. g., 



the well-known iron of Ovifak, in Greenland is of vol- 
canic origin. The materials in the gaseous interior of 
the earth will, owing to their high density, behave in 
chemical and physical respects like liquids. As sub- 
stances like iron will, also at very high temperatures, 
have a far higher specific gravity than their oxides, and 
these again have a higher gravity than their silicates, 
we have to assume that the gases in the core of the earth 
will almost exclusively be metallic, that the outer por- 
tions of the core will contain essentially oxides, and those 
farther out again mostly silicates. 

The fused magma will, on penetrating in the shape of 
batholithes into the upper layers, probably be divided 
into two portions, of which one, the lighter and gaseous, 
will contain water and substances soluble in it; while 
the other, heavier portion, will essentially consist of sili- 
cates with a lower percentage of water. The more fluid 
portion, richer in water, will be secreted in the higher 
layers, will penetrate into the surrounding sedimentary 
strata, especially into their fissures, and will fill them 
with large crystals, often of metallurgical value e.g., of 
the ores of tin, copper, and other metals, while the water 
will slowly evaporate through the superposed strata. The 
more viscid and sluggish mass of silicates, on the other 
hand, will congeal, thanks to its great viscosity, to glass, 
or, when the cooling is very slow, to small crystals. 

We now turn to earthquakes. No country has been 
absolutely spared by earthquakes. In the districts bound- 
ing upon the Baltic, and especially in northern Russia, 
they have, however, been of a quite harmless type. The 
reason is that the earth-crust there has been lying un- 
disturbed for long geological epochs and has never been 
fractured. The comparatively severe earthquake which 

shook the west coast of Sweden on October 23, 1904, to 



an unusually heavy degree, without, however, causing 
any noteworthy damage (a few chimneys were knocked 
over), was caused by a fault of relatively pronounced 
character for those districts in the Skager-Rack a con- 
tinuation of the deepest fold in the bottom of the North 
Sea, the so-called Norwegian Trough, which runs parallel 
to the Norwegian coast. In Germany, the Vogtland and 
the districts on both sides of the middle Rhine have fre- 
quently been visited by earthquakes. Of other European 
countries, Switzerland, Spain, Italy, and the Balkan Pen- 
insula, as well as the Karst districts of Austria, have 
often suffered from earthquakes. 

According to the committee appointed by the British 
Association for the investigation of earthquakes a com- 
mittee which has contributed a great deal to our knowl- 
edge of these great natural phenomena earthquakes of 
some importance emanate from certain centres which 
have been indicated on the subjoined map (Fig. 10). The 
most important among these regions comprises Farther 
India, the Sunda Isles, New Guinea, and Northern Aus- 
tralia; it is marked on the map by the letter F. From 
this district have emanated in the six-year period 1899- 
1904 no fewer than 249 earthquakes, which have been 
recorded in many observatories far removed from one 
another. This earthquake centre F is closely related to 
the one marked E, in Japan, from which 189 earthquakes 
have proceeded. Next to this comes the extensive dis- 
trict K with 174 earthquakes, comprising the most im- 
portant folds in the crust of the Old World, including the 
mountain chains from the Alps to the Himalaya. This 
district is interesting, because it has been disturbed by 
a great many earthquakes, although it is almost entirely 
situated on the Continent. After that we have the dis- 
districts A, B, C, with 125, 98, and 95 earthquakes. They 



are situated near lines of fracture in the earth -crust 
along the American coast of the Pacific Ocean and the 
Caribbean Sea. District D, with 78 earthquakes, is simi- 
larly situated. The three last - mentioned districts, B, 
C, D, as well as G, between Madagascar and India, with 
85 earthquakes, all seem to be surpassed by the district 
H in the eastern Atlantic, with its 107 earthquakes. 
These latter are, however, relatively feeble, and we owe 
their accurate records probably to the circumstances that 
a great many earthquake observatories are situated with- 
in the immediate surroundings of this district. The same 
may be said of the district I, or Newfoundland, which is 
not characterized by many earthquakes, and of the dis- 
trict J, between Iceland and Spitzbergen, with 31 and 19 

Fig. 10. Chief earthquake centres, according to the British 
Association Committee 

earthquakes respectively. The last on the list used to 
be the district L, situated about the South Pole, with only 

eight earthquakes. This small number is probably inere- 



ly due to the want of observatories in those parts of the 
earth. Another district, M, has finally been added, 
which extends to the southwest from,New Zealand. No 
fewer than 75 intense earthquakes were recorded between 
March 14 and November 23, 1903, by the Discovery Expe- 
dition,^ 70 sou them latitude and 178 eastern longitude. 

Earthquakes commonly occur in swarms or groups. 
Thus, more than 2000 shocks were counted on Hawaii 
in March, 1868. During the earthquakes which de- 
vastated the district of Phokis, in Greece, in 1870-73, 
shocks succeeded one another for a long time at intervals 
of three seconds. During the whole period of three and 
a half years about half a million shocks were counted, 
and, further, a quarter of a million subterranean reports 
which were not accompanied by noticeable concussions. 
Yet of all these shocks only about 300 did noteworthy 
damage, and only 35 were considered worth being re- 
ported in the newspapers. The concussion of October 
23, 1904, belonged to a group which lasted from October 
10 to October 28, and in which numerous small tremors 
were noticed, especially on October 24 and 25. The 
earthquake of San Francisco commenced on April 18, 
1906, at 5 hrs. 12 min. 6 sec. A.M. (Pacific Ocean time), 
and ended at 5 hrs. 13 min. 11 sec., lasting therefore 
1 minute and 5 seconds. Twelve smaller shocks s,ucceed- 
ed in the following hour. Before 6 hrs. 52 min. P.M., 
nineteen further concussions were counted, and various 
smaller shocks succeeded in the following days. 

With such groups of earthquakes weaker tremors usually 
precede the violent destructive shocks and give a warn- 
ing. Unfortunately this is not always so, and no warning 
was given by the earthquakes which destroyed Lisbon in 
1755 and Caracas in 1812, nor by those which devastated 
Agram in 1880, nor, finally, in the case of the San Fran- 
3 23 


cisco disaster. A not very severe earthquake without 
feebler precursors befell Ischia in 1881, while the violent 
catastrophe which devastated this magnificent island in 
1883 was heralded by several warnings. As in San Fran- 
cisco and Chili in 1906, less violent concussions generally 
succeed the destructive shocks. Earthquakes like that 
of Lisbon in 1755, consisting of a single shock, are very 
rare. * 

The violent concussions often produce large fissures 
in the ground. Such were noticed in several places at 
San Francisco. One of the largest fissures known, that of 
Midori, in Japan, was caused by the earthquake of Oc- 
tober 20, 1891. It left a displacement of the ground 
ranging up to 6 in. (20 ft.) in the vertical and 4 m. 
(13 ft.) in the horizontal direction. This crack had a 
length of not less than 65 km. (40 miles). Extensive 
fissures were also formed by the earthquakes of Calabria, 
in 1783, at Monte San Angelo, and in the sandstones of the 
Balpakram Plateau in India, in 1897. In mountainous 
districts falls of rock are a frequent consequence of the 
formation of fissures and earthquakes. A large number 
of rocks fell in the neighborhood of Delphi during the 
Phokian earthquake. On January 25, 1348, an earth- 
quake sent down a large portion of Mount Dobratsch (in 
the Alps of Villach, in Carinthia, which is now much fre- 
quented by tourists) and buried two towns and seventeen 
villages. The earthquake of April 18, 1906, in California 
started from a crack which extends from the mouth of 
Alder Creek, near Point Arena, running parallel with the 
coast-line mostly inland, then entering the sea near San 
Francisco, and turning again inland between Santa Cruz 
and San Jose, finally proceeding via Chittenden up to 
Mount Pinos, a distance of about 600 km. (400 miles), 
in the direction of N. 35 W. to S. 35 E. Along this 



Fig. 11. Clefts in Valentia Street, San Francisco, after the 
earthquake of 1906 

crack the two masses of the earth have been displaced so 
that the ground situated to the southwest of the fissure 
has been moved by about 3 m. (10 ft.), and in some 
spots even by 6 m. (20 ft.) towards the northwest. In 
some localities in Sonoma and Mendocino counties the 
southwestern part has been raised, but nowhere by more 
than 1.2 m. (4 ft.). This is the longest crack which has 
ever been noticed in connection with an earthquake. 

The earthquake over, the ground does not always return 
to its original position, but remains in a more or less wavy 
condition. This can most easily be observed in districts 
where streets or railways cross the ground. It is report- 
ed, for instance, that the track of the tramway-lines in 
Market Street, the chief thoroughfare of San Francisco, 
formed large wavelike curves after the earthquake. 

As a consequence of the displacements in the interior of 
the earth and of the formation of fissures, river courses are 
changed, springs become exhausted, and new springs arise. 
That was the case, for instance, in California in 1906. The 



ground water often rushes out with considerable violence, 
tearing with it sand and mud and stones, and piling them 
up, occasionally forming little craters (Fig. 12). Extensive 
floods may also be caused on such occasions. By such a 
flood the ancient Olympia was submerged under a layer 
of river sand which for some time preserved from destruc- 
tion the ancient Greek masterpieces of art among them 
the famous statue of Hermes. The floods afterwards re- 
ceded, and the treasures of ancient Olympia CuulJ be 

Like the natural water channels and arteries in the in- 
terior of the earth, water mains are displaced by the con- 
cussions. The direct damage caused by the floods is often 
less important than the damage due to the impossibility 
of extinguishing the fires which follow the destruction of 
the buildings. It was the fires that did most of the enor- 
mous material damage in the destruction of San Fran- 

Still greater devastation is wrought by the ocean waves 
thrown up by earthquakes. We have already referred to 
the flood of Lisbon in 1755, which was felt on the western 
coast of Norway and Sweden. Another wave, in 1510, de- 
voured 109 mosques and 1070 houses in Constantinople. 
Another wave, again, invaded Kamaishi, in Japan, on June 
15, 1896, swept away 7600 houses and killed 27,000 people. 

We have repeatedly alluded to the disastrous flood- 
wave of Krakatoa of 1883. This wave traversed the 
whole of the Indian Ocean, passing to the Cape of Good 
Hope and Cape Horn, and travelled round half the globe 
afterwards. Even more remarkable was the aerial wave, 
which spread like an explosion wave. 

While the most violent cannonades are rarely heard 
for more than 150 km. (95 miles) in a single case 
at a distance of 270 km. (170 miles) the eruption of 



Krakatoa was heard at Alice Springs, at a distance of 
3600 kilometres, and on the island of Rodriguez, at 
almost 4800 km. (3000 miles). The barographs of the 
meteorological stations first marked a sudden rise and 
then a decided sinking of the air pressure, succeeded by a 
few smaller fluctuations. These air pulses were repeated 
in some places as many as seven times. We may therefore 
assume that the aerial wave passed these places three 

Fig. 12. Sand craters and fissures, produced by the Corinth earth- 
quake of 1861. In the water, branches of flooded trees 

times in the one direction, and three times in the other, 
travelling round the earth. The velocity of propagation 
of this wave was 314.2 m. (1030 ft.) per second, corre- 
sponding to a temperature of 27 Cent. (17 F.) which 
prevails at an altitude of about 8 km. (5 miles) above the 



earth's surface, at which altitude this wave may have 

Within the last decade a peculiar phenomenon (leading 
to what is designated variation of latitudes) has been 
studied. The poles of the axis of the earth appear to 
move in a very irregular curve about their mean axis. 
The movement is exceedingly small. The deviation of the 
North Pole from its mean position does not amount to more 
than 10 m. (about 33 ft.). It has been believed that these 
motions of the North Pole are subject to sudden fluctua- 
tions after unusually violent earthquakes, especially when 
such concussions follow at rapid intervals. That would 
give us, perhaps more than any other observation, an 
idea of the force of earthquakes, since they would appear 
to be able to disturb the equilibrium of the whole mass 
of our globe. 

A severely felt effect of earthquakes, though most peo- 
ple perhaps pay little attention to it, is the destruction of 
submarine cables. The gutta-percha sheaths of cables are 
frequently found in a fused condition, suggesting volcanic 
eruptions under the bottom of the sea. We take care now 
to avoid earthquake centres in laying telegraphic cables. 
Their positions have been ascertained by the most modern 
investigations (see Fig. 10). 

People have always been inclined to look for a connec- 
tion between earthquakes and volcanic eruptions. The 
connection is unquestionable in a large number of violent 
earthquakes. In order to establish it, the above-men- 
tioned committee of the British Association has compiled 
the following table of the history of the earthquakes of 
the Antilles: 

1692. Port Royal, Jamaica, destroyed by an earthquake; 
land sinking into the sea. Eruption on St. Kitts. 



1718. Terrible earthquake on St. Vincent, followed by an 

1766-67. Great shocks in northeastern South America, in 
Cuba, Jamaica, and the Antilles. Eruption on Santa Lucia. 

1797. Earthquake in Quito, loss of 40,000 lives. Concus- 
sions in the Antilles, eruption on Guadeloupe. 

1802. Violent shocks in Antigua. Eruption on Guadeloupe. 

1812. Caracas, capital of Venezuela, totally destroyed by 
earthquake. Violent shocks in the Southern States of North 
America, commencing on November 11, 1811. Eruptions on St. 
Vincent and Guadeloupe. 

1835-36. Violent concussions in Chili and Central America. 
Eruption on Guadeloupe. 

1902. April 19. Violent shocks, destroying many towns of 
Central America. Mont Pelee, on Martinique, in activity. 
Eruption on May 3. Submarine cables break, sea recedes. 
Renewed violent movements of the sea on May 8, 19, 20. Erup- 
tion on St. Vincent, cable destroyed on May 7. Violent erup- 
tion of Mont Pelee on May 8. Destruction of St. Pierre. 
Numerous smaller earthquakes. 

This table distinctly marks the restless state of affairs 
in that part of the earth, and how quiet and safe matters 
are comparatively in old Europe, especially in the north. 
Some parts of Central America are so persistently visited 
by earthquakes that one of them, Salvador, has been 
christened " Schaukelmatte." It is not saying too much 
to assert that the earth is there incessantly trembling. 
Other districts which are very frequently visited are the 
Kuriles and Japan, as well as the East Indian 'islands. 
In all these countries the crust of the earth has been broken 
and folded within comparatively* recent epochs (chiefly in 
the Tertiary age) by numerous fissures, and their com- 
pression is still going on. 

The smaller earthquakes, of which not less than 30,000 
are counted in the course of a year, do not stand in any 
closer relation to volcanic eruptions. This is also the 


Fig. 13. Earthquake lines in lower Austria 


case for a number of large earthquakes, among which we 
have to count the San Francisco earthquake. 

It is assured with good reason that earthquakes are 
often produced at the bottom of the sea, where there is a 
strong slope, by slips of sedimentary strata which have 
been washed down from the land into the sea in the course 
of centuries. Milne believes that the seaquake of Kamai- 
shi 1 of June 15, 1896, was of this character. Concussions 
may even be promoted by the different loading of the 
earth resulting from the fluctuations in the pressure of 
the air above it. 

Smaller, though occasionally rather violent, earthquakes 
are not infrequent in the neighborhood of Vienna. On the 
map (Fig. 13) we see three lines. The line A B is called 
4 the thermal line, because along it a number of hot springs, 
the thermaB of Meidling, Baden, Voslau, etc., are located, 
which are highly valued ; the other line B C is called the 
Kamp line, because it is traversed by the river Kamp; 
and the third B F is called the Miirz line, after the river 
Miirz. The main railway- track between Vienna and Bruck 
follows the valleys of A B and E F. 

These lines, which probably correspond to large fissures 
in the earth-crust, are known as sources of numerous 
earthquakes. The district about Wiener Neustadt, where 
the three lines' intersect, is often shaken by violent earth- 
quakes ; some of their dates have been marked on the map. 

The curve which is indicated by the letters X X on the 
map marks the outlines of an earthquake which started 
on January 3, 1873, from both sides of the Kamp line. 
It is striking to see how the earthquake spread in the 
loose ground of the plain between St. Polten and Tulln, 
while the masses of rock situated to the northwest and 
southeast formed obstacles to the propagation of the 
earthquake waves. 



Similar conclusions have been deduced from the study 
of the spreading of the waves which destroyed Charles- 
ton, South Carolina, in 1886. Twenty-seven lives were 
destroyed by this shock. It was the most terrible 
earthquake that ever visited the United States be- 
fore the year 1906. In the Charleston concussion the 
Alleghany Mountains proved a powerful bar against the 

g 14. Library building of Leland Stanford Junior University, in 
California, after the earthquake of 1906. The photograph shows the 
great strength of iron structures in comparison to the strength of 
brickwork. The effect of the earthquake on wooden structures 
can be seen in Fig. 11 



further propagation of the shocks, which all the more 
easily travelled in the loose soil of the Mississippi Valley. 
In San Francisco, likewise, the worst devastation fell upon 
those parts of the town which had been built upon the 
loose, partly made ground in the neighborhood of the 
harbor, while the buildings erected on the famous moun- 
tain ridges of San Francisco suffered comparatively little 
damage, in so far as they were not reached by the destruc- 
tive fires. As regards the destructive effects of the earth- 
quake in San Francisco, the building-ground of that city 
has been divided into four classes (the first is the safest, 
the last the most unsafe) namely : 1 . Rocky soil. 2. Valleys 
situated between rocks and filled up by nature in the course 
of time. 3. Sand-dunes. 4. Soil created by artificial filling up. 
This latter soil "behaved like a semiliquid jelly in a dish," 
according to the report of the Earthquake Commission. 

For similar reasons the sky-scrapers, constructed of 
steel on deep foundations, stood firmest. After them 
came brick houses, with well-joined and cemented walls 
on deep foundations. The weakness of wooden houses 
proved mainly due to the poor connection of the beams, 
a defect which might easily be remedied. The superiority 
of the steel structure will be apparent from the illustra- 
tions (Figs. 11 and 14). 

The spots situated just over the crack, of which we 
spoke on page 25, suffered the most serious damage. Next 
to them, devastation befell especially localities which, like 
Santa Rosa, San Jose, and Palo Alto with Leland Stan- 
ford Junior University, are situated on the loose soil of the 
valley, whose deepest portions are covered by the bay of 
San Francisco. The splendidly endowed California Uni- 
versity, in Berkeley, and the famous Lick Observatory, 
both erected on rocky ground, fortunately escaped with- 
out any notable damage. 



The map sketch (Fig. 15) by Suess represents the earth- 
quake lines of Sicily and Calabria. These districts have, 
as mentioned before, been devastated by severe earth- 
quakes, of which the most terrible occurred in the year 
1783, and again in 1905 and 1907. They have also been 
the scene of many smaller concussions. 

The bottom of the Tyrrhenian Sea between Italy, 
Sicily, and Sardinia has been lowered in rather recent ages 

Fig. 15. Earthquake lines in the Tyrrhenian depression 

and is still sinking. We notice on the map five dotted linos, 
corresponding to cracks in the crust of the earth. These 
lines would intersect in the volcanic district of the Lipari 
Islands. We further see a dotted circular arc correspond- 
ing to a fissure which is regarded as the source of the Cala- 

brian earthquakes of 1783, 1905, and 1907. The earth- 



crust behaved somewhat after the manner of a window- 
pane which was burst by a heavy impact from a point 
corresponding to the Island of Lipari. From this point 
radiate lines of fracture, and fragments have been broken 
off from the earth-crust by arc-shaped cracks, The vol- 
cano Etna is situated on the intersection of the radial and 
circular fissures. 

In recognition of the high practical importance of earth- 
quake observations, seismological stations have in recent 
days been erected in many localities. At these observatories 
the earthquakes are recorded by pendulums whose styles 
draw lines on tapes of paper moved by clock-work. As long 
as the earth is quiet the drawn line is straight. When earth- 
quakes set in, the line passes into a wavy curve. As long as 
the movement of the paper is slow, the curve merely looks 
like a widened straight line. The subjoined illustration 
(Fig. 16) represents a seismogram taken at the station of 
Shide, on the Isle of Wight, on August 31, 1898. The 
earthquake recorded originated in the Centre G, in the 
Indian. Ocean. The origin has been deduced from the 

20. 36. 25. 
SO. 31. 21. | 20. 42. 

Fig. 16. Seismogram recorded at Shide, Isle of Wight, on 
August 31, 1898 

moments of arrival of the different waves at different 
stations. We notice on the seismogram a faint widening 
of the straight line at 20 hrs. 5 min. 2 sec. (8 hrs. 5 min. 
2 sec. P.M.). The amplitude of the oscillations then began 



to widen, and the heaviest concussions were noticed at 
20 hrs. 36 min. 25 sec., and 20 hrs. 42 min. 49 sec., 
after which the amplitudes slowly decreased with smaller 
shocks. The first shock of 20 hrs. 5 min. 2 sec. is called 
the preliminary tremor. This tremor passes through the 
interior of the earth at a velocity of propagation of 9.2 
km. (5 1 miles) per second. It would require twenty- three 
minutes to pass through the earth along a diameter. The 
tremor is very feeble, which is ascribed to the extraordi- 
narily great friction characteristic of the strongly heated 
gases which are confined in the interior of the earth. 
The principal violent shock at 20 hrs. 36 min. 25 sec. was 
caused by a wave travelling through the solid crust of the 
earth. The intensity of this shock is much less impaired 
than that of the just-mentioned tremor, and it travels 
with the smaller velocity of about 3.4 km. (2.1 miles) along 
the earth's surface. 

The velocity of propagation of concussion pulses has been 
calculated for a mountain of quartz, in which it would be 
3.6 km. (2.2 miles) per second, very nearly the same as the 
last-mentioned figure. We should expect this, since the 
firm crust of the earth consists essentially of solid silicates 
i.e., compounds of quartz endowed with similar prop- 

Measured at small distances from the origin, the veloc- 
ity of propagation of the wave appears smaller, and the 
first preliminary tremor is frequently not observed. The 
velocity may be diminished to 2 km. (1J miles) per second. 
The reason is that the pulse partly describes a curve in 
the more solid portions of the crust, and partly passes 
through looser strata, through which the wave travels at 
a much slower rate than in firm ground; for instance, at 
1.2 km. through loose sandstones, at 1.4 km. through the 
water of the ocean, and at 0.3 km. through loose sand. 



We recognize that it should be possible to calculate the 
distance between the point of observation and the origin 
of the earthquake from the data relating to the arrivals 
of the first preliminary tremor and of the principal shock 
of maximum amplitude. The violent shock is some- 
times repeated after a certain time, though with de- 
creased intensity. It has often been observed that 
this secondary, less violent, shock seems to have trav- 
elled all round the earth via the longest road between 
the origin and the point of observation, just like one 
portion of the aerial waves in the eruption of Krakatoa 
(compare page 27). The velocity of propagation of 
this secondary shock is the same as that of the princi- 
pal shock. 

Milne has deduced from his observations that, when the 
line joining the origin of the earthquake and the point of 
observation does not at its lowest level descend deeper 
than 50 km. below the surface of the earth, the pulse will 
travel undivided through the solid crust of the earth. For 
this reason we estimate the thickness of the solid crust at 
50 km. The value is in almost perfect agreement with 
the one which we had (on page 16) derived from the in- 
crease of temperature with greater depths. It should fur- 
ther be mentioned, perhaps, that the density of the earth 
in the vicinity has been determined from pendulum ob- 
servation, and that this density seems to be rather varia- 
ble down to the depths of 50 or 60 km., but to become 
more uniform at greater depths. These 50 or 60 km. 
(31 or 37 miles) would belong to the solid crust of tne 

The movement of earthquake shocks through the earth 
thus teaches us that the solid earth-crust cannot be very 
thick, and that the core of the earth is probably gaseous. 
The similar conclusions, to which these various consider- 



ations had led us, may therefore come very near the truth. 
A careful study of seismograms may, we hope, help us to 
learn more about the central portions of the earth, which 
at first sight appear to be absolutely inaccessible to scien- 
tific research. 



THERE is no more elevating spectacle than to contem- 
plate the sky with its thousands of stars on a clear night. 
When we send our thoughts to those lights glittering in 
infinite distance, the question forces itself upon us, whether 
there are not out there planets like our own that will sus- 
tain organic life. How little interest do we take in a barren 
island of the Arctic Circle, on which not a single plant will 
grow, compared to an island in the tropics which is teem- 
ing with life in its most wonderful variety ! The unknown 
worlds occupy our minds much more when we may fancy 
them inhabited than when we have to regard them as 
dead masses floating about in space. 

We have to ask ourselves similar questions with regard 
to our own little planet, the earth. Was it always covered 
with verdure, or was it once sterile and barren? And if 
that be so, what are the conditions under which the earth 
can fulfil its actual part of harboring organic life? That 
" the earth was without form" in the beginning is unques- 
tionable. It does not matter whether we assume that it 
was once all through an incandescent liquid, which may 
be the most probable assumption, or that it was, as 
Lockyer and Moulton think, formed by the accumulation 
of meteoric stones which became incandescent when ar- 
rested in their motion. 

4 39 


We have seen that the earth probably consists of a mass 
of gas encased within a shell which is solid on the outside 
and remains a viscid liquid on the inner side. We pre- 
sume with good reason that the earth was originally a 
mass of gas separated from the sun, which is still in the 
same state. By radiation into cold space the sphere of gas 
which, on the whole, would behave as our sun does now, 
would gradually loss its high temperature, and finally a solid 
crust could form on its surface. Lord Kelvin has calculated 
that it would not require more than one hundred years 
before the temperature of this crust would sink to 100. 
Supposing, even, that Kelvin's calculations should not 
quite be confirmed, we may yet maintain that not many 
thousands of years would have elapsed from the time when 
the earth assumed its first crust at about 1000 till the 
age when this temperature had fallen below 100 (212 F.). 
Living beings certainly could not exist so long, since the 
albumen of the cells would at once coagulate at the tem- 
perature of boiling water, like the white of an egg. Yet 
it has been reported that some of the hot springs of New 
Zealand contain algae, although at a temperature of over 
80. When I went to Yellowstone Park to inquire into the 
correctness of this statement, I found that the algae existed 
only at the edge of the hot springs, where the temperature 
did not exceed 60 (140^ yF.). The famous American 
physiologist Loeb states that we do not meet with algae in 
hot springs at temperatures above 55. 

Since, now, the temperature of the earth-crust would 
much more quickly sink from 100 to 55 than it had fallen 
from 1000 to 100, we may imagine that only a few thou- 
sands of years may have intervened between the formation 
of the first crust of the earth and the cooling down to a 
temperature such as would sustain life. Since that time the 
temperature has probably never been so low that the 



larger portion of the earth's surface would not have been 
able to support organisms, although there have been 
several glacial ages in which the arctic districts inacces- 
sible to life must have extended much farther than at 
present. The ocean will also have been free of ice over 
much the greatest portion of its surface at all times, and 
may therefore have been inhabited by organisms in all 
ages. The interior of the earth cools continually, though 
slowly, because heat passes from the inner, warmer por- 
tions to the other, cooler portions through the crust of the 

The earth is able to serve as the abode of living Beings 
because its outer portions are cooled to a suitable tem- 
perature (below 55) by radiation, and because the cool- 
ing does not proceed so far that the open sea would con- 
tinually be frozen over, and that the temperature on the 
Continent would always remain below freezing-point. We 
owe this favorable intermediate stage to the fact that 
the radiation from the sun balances the loss of heat by 
radiation into space, and that it is- capable of maintaining 
the greater portion of the surface of the earth at a tem- 
perature above the freezing-point of water. The temper- 
ature conditioning life on a planet is therefore maintained 
only because, on the one side, light and heat are received 
by radiation from the sun in sufficient quantities, while on 
the other side an equivalent radiation of heat takes place 
into space. If the heat gain and the heat loss were not 
to balance each other, the term of suitable conditions would 
not last long. The temperature of the earth-crust could 
sink in a few hundreds or thousands of years from 1000 
to 100, because when the earth was at this high tempera- 
ture its radiation into space predominated over the radia- 
tion received from the sun. On the other hand, about a 

hundred million years have passed, according to Joly, since 



the age when the ocean originated. The temperature of 
the earth, therefore, required this long space of time in order 
to cool down from 365 (at which temperature water vapor 
can first be condensed to liquid water) to its present tem- 
perature. The cooling afterwards proceeded at a slower 
rate, because the difference between the radiations inward 
and outward was lessened with the diminishing tempera- 
ture of the earth. Various methods have been applied in 
estimating these periods. Joly based his estimate on the 
percentage of salt in the sea and in the rivers. If we cal- 
culate how much salt there is in the sea, and how much 
salt the rivers can supply to it in the course of a year, we 
arrive at the result that the quantity of salt now stored in 
the ocean might have been supplied in about a hundred 
million years. 

We arrive at still higher numbers when we calculate the 
time which must have elapsed during the deposition of 
all the stratified and sedimentary layers. Sir Archibald 
Geikie estimates the total thickness of those strata, sup- 
posing them to have been undisturbed, at 30,000 m. 
(nearly 20 miles). He concludes, further, from the ex- 
amination of more recent strata, that every stratum one 
metre in thickness must have required from 3000 to 20,000 
years for its formation. We should, therefore, have to al- 
low a space of from ninety to six hundred million years for 
the deposition of all the sedimentary strata. The Finnish 
geologist Sederholm even fixes the time at a thousand 
million years. 

Another method again starts from the consideration 
that, while the temperature of the surface of the earth 
remains fairly steady owing to the heat exchange between 
solar radiation and terrestrial radiation into space, the in- 
terior of the earth must have shrunk with the cooling. 
How far this shrinkage extends we may estimate from the 



formation of the mountain chains which, according to 
Rudzki, cover 1.6 per cent, of the earth's surface. The 
earth's radius should consequently have contracted by 
about 0.8 per cent., corresponding to a cooling through 
about 300, which would require two thousand million years. 

Quite recently the renowned physical chemist Ruther- 
ford has expounded a most original method of estimating 
the age of minerals. Uranium and thorium are supposed 
to produce helium by their slow dissociation, and we know 
how much helium is produced from a certain quantity of 
uranium or thorium in a year. Now Ramsay has deter- 
mined the percentage of helium in the uranium mineral 
fergusonite and in thorianite. Rutherford then calculates 
the time which would have passed since the formation of 
these minerals. He demands at least four hundred million 
years, "for very probably some helium has escaped from 
the minerals during that time." Although this estimate 
is very uncertain, it is interesting to find that it leads to 
an age for the solid earth-crust of the same order of magni- 
tude as the other methods. 

During this whole epoch of almost inconceivable length 
of between one hundred million and two thousand million 
years, organisms have existed on the surface of the earth 
and in the sea which do not differ so very much from 
those now alive. The temperature of the surface may 
have been higher than it is at present; but the differ- 
ence cannot be very great, and will amount to 20 Cent. 
(36 F.) at the highest. The actual mean temperature of 
the surface of the earth is 16 Cent. (61 F.). It varies 
from about 20 Cent. (-4F.) at the North Pole, and 
-10 Cent. ( + 14 F.) at the South Pole to 26 Cent. (79 
F.) in the tropical zone. The main difference between the 
temperatures of the earth's surface in the most remote 
period from which fossils are extant and the actual state 



rather seems to be that the different zones of the earth 
are now characterized by unequal temperatures, while in 
the remote epochs the heat was almost uniformly dis- 
tributed over the whole earth. 

The condition for this prolonged, almost stationary state 
was that the gain of heat of the earth's surface by radiation 
from the sun and the loss of heat by radiation into space 
nearly balanced each other. That the replenishing supply 
by radiation from an intensely hot body in our case the 
sun is indispensable for the existence of life will be evident 
to everybody. Not everybody may, however, have con- 
sidered that the loss of heat into cold space or into colder 
surroundings is just as indispensable. To some people, 
indeed, the assumption that the earth as well as the sun 
should waste the largest portions of their vital heat as radi- 
ation into cold space appears so unsatisfactory that they pre- 
fer to believe radiation to be confined to radiation between 
celestial bodies; there is no radiation into space, in their 
opinion. All the solar heat would thus benefit the planets 
and the moons in the solar system, and only a vanishing 
portion of it would fall upon the fixed stars, because their 
visual angles are so small. If that were really correct, the 
temperature of the planets would rise at a rapid rate until 
it became almost equal to that of the sun, and all life would 
become impossible. We are therefore constrained to ad- 
mit that " things are best as they arc," although the great 
waste of solar heat certainly weakens the solar energy. 

The opinion that all the solar heat radiated into infinite 
space is wasted, starts moreover from a hypothesis which is 
not proved, and which is highly improbable namely, that 
only an extremely small portion of the sky is covered with 
celestial bodies. That might certainly be correct if we 
assumed, as has formerly been done, that the majority 
of the celestial bodies must be luminous. We do not 



possess, however, any reliable knowledge of the number 
and size of the dark celestial bodies. In order to account 
for the observed movements of different stars, it has 
been thought that there must be in the neighborhood of 
some of them dark stars of enormous size whose masses 
would surpass the mass of our sun, or, at least, be equal 
to it. But the largest number of the dark celestial bodies 
which hide the rays from the stars behind them probably 
consist of smaller particles, such as we observe in meteors 
and in comets, and to a large extent of so-called cosmical 
dust. The observations of later years, by the aid of 
most powerful instruments, have shown that so-called 
nebulse and nebulous stars abound throughout the heav- 
ens. In their interior we should probably find accumu- 
lations of dark masses. 

The light intensity of most of the nebulae is, moreover, 
far too weak to permit of their being perceived. We 
have, therefore, to imagine that there are bodies all 
through infinite space, and about as numerous as they 
are in .the immediate neighborhood of our solar system. 
Thus every ray from the sun, of whatever direction, would 
finally hit upon some celestial body, and nothing would 
be lost of the solar radiation, nor of the stellar radiation. 

As regards the radiation-heat exchange, the earth 
might be likened to a steam-engine. In order that the 
steam-engine shall perform useful work, it is necessary 
not only that the engine be supplied with heat of high 
temperature from a furnace and a boiler, but also that the 
engine be able to give its heat up again to a heat reservoir 
of lower temperature a condenser or cooler. It is only 
by transferring heat from a body of higher temperature 
to another body of lower temperature that the engine 
can do work. In a similar way no work can be done 
on the earth, and no life can exist, unless heat be conferred 



by the intermediation of the earth from a hot body, the 
sun, to the colder surroundings of universal space i. e., 
to the cold celestial bodies in it. 

To a certain extent the temperature of the earth's sur- 
face, as we shall presently see, is conditioned by the prop- 
erties of the atmosphere surrounding it, and particularly 
by the permeability of the latter for the rays of heat. 

If the earth did not possess an atmosphere, or if this 
atmosphere were perfectly diathermal i.e., pervious 
to heat radiations we should be able to calculate the 
mean temperature of the earth's surface, given the in- 
tensity of the solar radiation, from Stefan's law of the 
dependence of heat radiation bn its temperature. Start- 
ing from the not improbable assumption that, at a mean 
distance of the earth from the sun, the solar rays would 
send 2.5 gramme-calories per minute to a body of cross 
section of 1 sq. centimetre at right angles to the rays of 
the sun, Christiansen has calculated the mean tempera- 
tures of the surfaces of the various planets. The follow- 
ing table gives his figures, and also the mean distances 
of the planets from the sun, in units of the mean distance 
of the earth from the sun, 149.5 million kin. (nearly 93 
million miles) : 


Radius Mass 



to See 

According to See 

Mercury... . 




+ 17S(332) 
+ 65 
+ 6.5 
+ 6.5(105) 
- 37 
- 147 
- 180 
- 207 
- 221 
+ 6200 




Saturn . . . 
Uranus .... 
Neptune . . 



In the case of Mercury, I have added another figure, 
332. Mercury always turns the same side to the sun, 
and the hottest point of this side would reach a tempera- 
ture of 397; its mean temperature, according to my cal- 
culation, is 332, while the other side, turned away from 
the sun, cannot be at a temperature much above absolute 
zero, 273. I have made a similar calculation for the 
moon, which turns so slowly about its axis (once in 
twenty-seven days) that the temperature on the side 
illuminated by the sun remains almost as high (106) 
as if the moon were always turning the same face to the 
sun. The hottest point of this surface would attain a 
temperature of 150, while the poles of the moon and that 
part of the other side which remains longest without 
illumination can, again, not be much above absolute zero 
temperature. This estimate is in fair agreement with 
the measurements made of the lunar radiation and the 
temperature estimate based upon it. The first measure- 
ment of this kind was made by the Earl of Rosse. He 
ascertained that the moon disk as illuminated by the 
sun that is to say, the full moon would radiate as 
much heat as a black body of the temperature 110 Cent. 
(230 F.). A later measurement by the American Very 
seems to indicate that the hottest point of the moon is 
at about 180, which would be 30 higher than my esti- 
mate. In the cases of the moon and of Mercury, which 
do not possess any atmosphere to speak of, this calcula- 
tion may very fairly agree with the actual state of affairs. 

The temperature of the planet Venus would be about 
65 Cent. (149 F.) if its atmosphere were perfectly trans- 
parent. We know, however, that dense clouds, prob- 
ably of water drops, are floating in the atmosphere of 
this planet, preventing us from seeing its land and 
water surfaces. According to the determinations made 



by Zollner and others, Venus would reflect not less than 
76 per cent, of the incident light of the sun, and the planet 
would thus be as white as a snow-ball. The rays of heat 
are not reflected to the same extent. We may estimate 
that the portion of heat absorbed by the planet is about 
half the incident heat. The temperature of Venus will 
therefore be reduced considerably, but it is partly aug- 
mented again by the protective action of this atmosphere. 
The mean temperature of Venus may, hence, not differ 
much from the calculated temperature, and may amount 
to about 40 (104 F.). Under these circumstances the 
assumption would appear plausible that a very consid- 
erable portion of the surface of Venus, and particularly 
the districts about the poles, would be favorable to or- 
ganic life. 

Passing to the earth, we find that the temperature- 
reducing influence of the clouds must be strong. They 
protect about half of the earth's surfa'ce (52 per cent.) 
from solar radiation. But even with a perfectly clear 
sky, not all the light from the sun really reaches the 
earth's surface; for finely distributed dust is floating 
even in the purest air. I have estimated that this 
dust would probably absorb 17 per cent, of the solar 
heat. Clouds and dust would therefore together deprive 
the earth of 34 per cent, of the heat sent to it, which 
would lead to a reduction of the temperature by about 
28. Dust and the water-bubbles in the clouds also 
prevent the radiation of heat from the earth, so that 
the total loss of heat to be charged to clouds and dust 
will amount to about 20 (36 F.). 

It has now been ascertained that the mean temperature 
of the earth is 16 (61 F.), instead of the calculated 6.5 
(43.7 F.). Deducting the 20 due to the influence of 
dust and clouds, we obtain 14 (7 F.), and the ob- 



served temperature would therefore be higher than the 
calculated by no less than 30 (54 F.). The discrep- 
ancy is explained by the heat-protecting action of the 
gases contained in the atmosphere, to which we shall 
presently refer (page 51). 

There are but few clouds on Mars. This planet is 
endowed with an atmosphere of extreme transparency, 
and should therefore have a high temperature. Instead 
of the temperature of -37 (35 F.), calculated, the 
mean temperature seems to be +10 ( + 50 F.). During 
the winter large white masses, evidently snow, collect 
on the poles of Mars, which rapidly melt away in 
spring and change into water that appears dark to us. 
Sometimes the snow-caps on the poles of Mars disappear 
entirely during the Mars summer; this never happens 
on our terrestrial poles. The mean temperature of Mars 
must therefore be above zero, probably about +10. 
Organic life may very probably thrive, therefore, on 
Mars. It is, however, rather sanguine to jump at the 
conclusion that the so-called canals of Mars prove its 
being inhabited by intelligent beings. Many people re- 
gard the "canals" as optical illusions; Lowell's photo- 
graphs, however, do not justify this opinion. 

As regards the other large planets, the temperatures 
which we have calculated for them are very low. This 
calculation is, however, rather illusory, because these 
planets probably do not possess any solid or liquid sur- 
face, but consist altogether of gases. Their densities, at 
least, point in this direction. In the case of the inner 
planets, Mars and our moon included, the density is rather 
less than that of the earth. Mercury stands last among 
them, with its specific gravity of 0.564. There follows a 
great drop in the specific gravities of the outer large plan- 
ets. Saturn, with a density of 0.116, is last in this order; 



the densities of the two outermost planets lie somewhat 
higher by 0.3 or 0.4 about but these last data are very 
uncertain. Yet these figures are of the same order of mag- 
nitude as that assumed for the sun 0.25 and we be- 
lieve that the sun, apart from the small clouds, is wholly 
a gaseous body. It is therefore probable that the outer 
planets, including Jupiter, will also be gaseous and be 
surrounded by dense veils of clouds which prevent our 
looking down into their interior. That view would con- 
tend against the idea that these planets can harbor any 
living beings. We could rather imagine their moons to 
be inhabited. If these moons received no heat from their 
planets, they would assume the above-stated tempera- 
tures of their central bodies. Looked at from our moon, 
the earth appears under a visual angle, 3.7 times as large 
as that of the sun. As the temperature of the sun has, 
from its radiation, been estimated at 6200 Cent., or 
6500 absolute, the moon would receive as much heat 
from the earth as from the sun, if the earth had a tem- 
perature of about 3100 Cent., or 3380 absolute. When 
the first clouds of water vapor were being formed in the 
terrestrial atmosphere, the earth's temperature was 
about 360, and the radiation from the earth to the 
moon only about 1.25- thousandth of that of the sun. The 
present radiation from the earth does not even attain 
one- twentieth of this value. It is thus manifest that 
the radiation from the earth does not play any part in 
the thermal household of the moon. 

The relations would be quite different if the earth had 
the 11.6 times greater diameter of Jupiter, or the diameter 
of Saturn, which is 9.3 times greater than its own. The 
radiation from the earth to the moon would then make 
up about a sixth or a ninth of the actual solar radiation, 

taking the temperature of the earth's surface at 360. 



We can easily calculate, further, that Jupiter and Saturn 
would radiate as much heat against a moon at a distance 
of 240,000 or 191,000 km. respectively (since the distance 
of the moon from the earth amounts to 384,000 km.) as the 
sun sends to Mars taking the temperature of those 
planets at 360 Cent. Now we find, near Jupiter as well 
as near Saturn, moons at the distances of 126,000 and 
186,000 km. respectively, which are smaller than those 
mentioned, and it is not inconceivable that these moons 
receive from their central bodies sufficient heat to render 
life possible, provided that they be enveloped by a heat- 
absorbing atmosphere. The conditions appear to be 
less favorable for the innermost satellites of Jupiter and 
Saturn. When their planets are shining at the maxi- 
mum brilliancy, their light intensity is only a sixth or a 
ninth of the solar light intensity, which upon these satel- 
lites is itself only one-twenty-seventh or one-ninetieth of 
the intensity on the earth. During the incandescence 
epoch of these planets their moons will certainly for 
some time have been suitable for the development of life. 
That the atmospheric envelopes limit the heat losses 
from the planets had been suggested about 1800 by the 
great French physicist Fourier. His ideas were further 
developed afterwards by Pouillet and Tyndall. Their 
theory has been styled the hot-house theory, because 
they thought that the atmosphere acted after the man- 
ner of the glass panes of hot-houses. Glass possesses 
the property of being transparent to heat rays of small 
wave lengths belonging to the visible spectrum; but it is 
not transparent to dark heat rays, such, for instance, as 
are sent out by a heated furnace or by a hot lump 
of earth. The heat rays of the sun now are to a large 
extent of the visible, bright kind. They penetrate 
through the glass of the hot-house and heat the earth 



under the glass. The radiation from the earth, on the 
other hand, is dark and cannot pass back through the 
glass, which thus stops any losses of heat, just as an over- 
coat protects the body against too strong a loss of heat 
by radiation. Langley made an experiment with a box, 
which he packed with cotton-wool to reduce loss by 
radiation, and which . he . provided, on the side turned 
towards the sun, with a double glass pane. He ob- 
served that the temperature rose to 113 (235 F.), while 
the thermometer only marked 14 or 15 (57 or 59 F.) 
in the shade. This experiment was conducted on Pike's 
Peak, in Colorado, at an altitude of 4200 m. (13,800 ft.), 
on September 9, 1881, at 1 hr. 4 min. p. M., and therefore 
at a particularly intense solar radiation. 

Fourier and Pouillet now thought that the atmosphere 
of our earth should be endowed with properties resem- 
bling those of glass, as regards permeability of heat. 
Tyndall later proved this assumption to be correct. The 
chief invisible constitutents of the air which participate 
in this effect are water vapor, which is always found in a 
certain quantity in the air, and carbonic acid, also ozone 
and hydrocarbons. These latter occur in such small 
quantities that no allowance has been made for them so 
far in the calculations. Of late, however, we have been 
supplied with very careful observations on the per- 
meability to heat of carbonic acid and of water vapor. 
With the help of these data I have calculated that if 
the atmosphere were deprived of all its carbonic acid 
of which it contains only 0.03 per cent, by volume the 
temperature of the earth's surface would fall by about 21. 
This lowering of the temperature would diminish the 
amount of water vapor in the atmosphere, and would 
cause a further almost equally strong fall of temperature. 

The examples, so far as they go, demonstrate that com- 



paratively unimportant variations in the composition of 
the air have a very great influence. If the quantity of 
carbonic acid in the air should sink to one-half its present 
percentage, the temperature would fall by about 4; a 
diminution to one-quarter would reduce the temperature 
by 8. On the other hand, any doubling of the per- 
centage of carbon dioxide in the air would raise the tem- 
perature of the earth's surface by 4; and if the carbon 
dioxide were increased fourfold, the temperature would 
rise by 8. Further, a diminution of the carbonic acid 
percentage would accentuate the temperature differences 
between the different portions of the earth, while an in- 
crease in this percentage would tend to equalize the tem- 

The question, however, is whether any such tempera- 
ture fluctuations have really been observed on the sur- 
face of the earth. The geologists would answer: yes. 
Our historical era was preceded by a period in which the 
mean temperature was by 2 (3.6 F.) higher than at 
present.. We recognize this from the former distribution 
of the ordinary hazel-nut and of the water-nut (Trapa 
nalans). Fossil nuts of these two species have been 
found in localities where the plants could not thrive 
in the present climate. This age, again, was preceded 
by an age which, we are pretty certain, drove the inhabi- 
tants of northern Europe from their old abodes. The 
glacial age must have been divided into several periods, 
alternating with intervals of milder climates, the so- 
called inter-glacial periods. The space of time which is 
characterized by these glacial periods, when the tem- 
perature according to measurements based upon the 
study of the spreading of glaciers in the Alps must 
have been about 5 (8 F.) lower than now, has been 

estimated by geologists at not less than 100,000 years. 



This epoch was preceded by a wanner age, in which the 
temperature, to judge from fossilized plants of those 
days, must at times have been by 8 or 9 (14 or 16 F.) 
higher than at present, and, moreover, much more uni- 
formly distributed over the whole earth (Eocene). Pro- 
nounced fluctuations of this kind in the climate have 
also occurred in former geological periods. 

Are we now justified in supposing that the percentage 
of carbon dioxide in the air has varied to an extent suffi- 
cient to account for the temperature changes? This 
question has been answered in the affirmative by Hog- 
bom, and, in later times, by Stevenson. The actual 
percentage of carbonic acid in the air is so insignificant 
that the annual combustion of coal, which has now (1904) 
risen to about 900 million tons and is rapidly increas- 
ing, 1 carries about one-seven-hundredth part of its per- 
centage of carbon dioxide to the atmosphere. Although 
the sea, by absorbing carbonic acid, acts as a regulator 
of huge capacity, which takes up about five-sixths of the 
produced carbonic acid, we yet recognize that the slight 
percentage of carbonic acid in the atmosphere may by 
the advances of industry be changed to a noticeable de- 
gree in the course of a few centuries. That would imply 
that there is no real stability in the percentage of carbon 
dioxide in the air, which is probably subject to consider- 
able fluctuations in the course of time. 

Volcanism is the natural process by which the greatest 
amount of carbonic acid is supplied to the air. Large 
quantities of gases originating in the interior of the earth 
are ejected through the craters of the volcanoes. These 
gases consist mostly of steam and of carbon dioxide, which 
have been liberated during the slow cooling of the silicates 

1 It amounted in 1890 to 510 million tons; in 1894, to 550; in 
1899, to 690; and in 1904, to 890 million tons. 



in the interior of the earth. The volcanic phenomena 
have been of very unequal intensity in the different phases 
of the history of the earth, and we have reason to surmise 
that the percentage of carbon dioxide in the air was 
considerably greater during periods of strong volcanic 
activity than it is now, and smaller in quieter periods. 
Professor Freeh, of Breslau, has attempted to demon- 
strate that this would be in accordance with geological 
experience, because strongly volcanic periods are dis- 
tinguished by warm climates, and periods of feeble vol- 
canic intensity by cold climates. The ice age in particular 
was characterized by a nearly complete cessation of vol- 
canism, and the two periods at the commencement and 
at the middle of the Tertiary age (Eocene and Miocene) 
which showed high temperatures were also marked by 
an extraordinarily developed volcanic activity. "This 
parallelism can be traced back into more remote epochs. 
It may possibly be a matter of surprise that the per- 
centage of carbon dioxide in the atmosphere should not 
constantly be increased, since volcanism is always pour- 
ing out more carbon dioxide into our atmosphere. There 
is, however, one factor which always tends to reduce the 
carbon dioxide of the air, and that is the weathering of 
minerals. The rocks which were first formed by the con- 
gelation of the volcanic masses (the so-called magma) con- 
sist of compounds of silicic acid with alumina, lime, mag- 
nesia, some iron and sodium. These rocks were gradu- 
ally decomposed by the carbonic acid contained in the 
air and in the water, and it was especially the lime, 
the magnesia, and the alkalies, arid, in some measure also 
the iron, which formed soluble carbonates. These car- 
bonates were carried by the rivers down into the seas. 
There lime and magnesia were secreted by the. animals 
and by the algse, and their carbonic acid became stored 
s 55 


up in the sedimentary strata. Hogbom estimates that the 
limestones and dolomites contain at least 25,000 times 
more carbonic acid than our atmosphere. Chamberlin has 
arrived at nearly the same figure from 20,000 to 30,000 ; 
he does not allow for the precambrian limestones. These 
estimates are most likely far too low. All the carbonic 
acid that is stored up in sedimentary strata must have 
passed through the atmosphere. Another process which 
withdraws carbonic acid from the air is the assimilation 
of plants. Plants absorb carbonic acid under secretion 
of carbon compounds and under exhalation of oxygen. 
Like the weathering, the assimilation increases with the 
percentage of carbonic acid. The Polish botanist E. 
Godlewski showed as early as 1872 that various plants (he 
studied Typha latifolia and Glyceria spectabilis with par- 
ticular care) absorb from the air an amount of carbonic 
acid which increases proportionally with the percentage 
of carbonic acid in the atmosphere up to 1 per cent., and 
that the assimilation then attains, in the former plant, a 
maximum at 6 per cent., and in the latter plant at 9 per 
cent. The assimilation afterwards diminishes if the car- 
bonic acid percentage is further augmented. If, therefore, 
the percentage of carbon dioxide be doubled, the absorp- 
tion by the plants would also be doubled. If, at the same 
time, the temperature rises by 4, the vitality will increase 
in the ratio of 1 : 1.5, so that the doubling of the carbon 
dioxide percentage will lead to an increase in the ab- 
sorption of carbonic acid by the plant approximately in 
the ratio of 1 : 3. The same may be assumed to hold for 
the dependence of the weathering upon the atmospheric 
percentage of carbonic acid. An increase of the carbon 
dioxide percentage to double its amount may hence be 
able to raise the intensity of vegetable life and the in- 
tensity of the inorganic chemical reactions threefold. 



According to the estimate of the famous chemist Lie- 
big, the quantity of organic matter (freed of water) which 
is produced by one hectare (2.5 acres) of soil, meadow- 
land, or forest is nearly the same, approximately 2.5 
tons per year in central Europe. In many parts of the 
tropics the growth is much more rapid; in other places, 
in the deserts and arctic regions, much more feeble. We 
may be justified in accepting Liebig's figure as an average 
for the firm land on our earth. Of the organic substances 
to which we have referred, and which mainly consist of 
cellulose, carbon makes up 40 per cent. Thus the actual 
annual carbon production by plants would amount to 
13,000 million tons i. e., not quite fifteen times more 
than the consumption of coal, and about one-fiftieth of 
the quantity of the carbon dioxide in the air. If, there- 
fore, all plants were to deposit their carbon in peat-bogs, 
the air would soon be depleted of its carbon dioxide. 
But it is only a fraction of one per cent, of the coal 
which is produced by plants that is stored up for the 
future in this way. The rest is sent back into the atmos- 
phere by combustion or by decay. 

Chamberlin relates that, together with five other 
American geologists, he attempted to estimate how long 
a time would be required before the carbon dioxide of 
the air would be consumed by the weathering of rocks. 
Their various estimates yielded figures ranging from 5000 
to 18,000 years, with a probable average of 10,000 years. 
The loss of carbonic acid by the formation of peat may 
be estimated at the same figure. The production of car- 
bonic acid by the combustion of coal would therefore 
suffice to cover the loss of carbonic acid by weathering 
and by peat formation seven times over. Those are 
the two chief factors deciding the consumption of car- 
bonic acid, and we thus recognize that the percentage of 



carbonic acid in the air must be increasing at a constant 
rate as long as the consumption of coal, petroleum, etc., 
is maintained at its present figure, and at a still more 
rapid rate if this consumption should continue to increase 
as it does now. 

This consideration enables us to picture to ourselves 
the possibility of the enormous plant-growth which must 
have characterized certain geological periods of our earth 
for instance, the carboniferous period. 

This period is known to us from the extraordinarily 
large number of plants which we find embedded in the 
clay of the swamps of those days. Those plants were 
slowly carbonized afterwards, and their carbon is in our 
age returned to its original place in the household of 
nature in the shape of carbonic acid. A great portion of 
the carbonic acid has disappeared from the atmosphere 
of the earth, and has been stored up as coal, lignite, peat, 
petroleum, or asphalt in the sedimentary strata. Oxygen 
was liberated at the same time, and passed into our at- 
mospheric sea. It has been calculated that the amount 
of oxygen in the air 1216 billion tons approximately 
corresponds to the mass of fossil coal which is stored up 
in the sedimentary strata. The supposition appears nat- 
ural, therefore, that all the oxygen of the air may have 
been formed at the expense of the carbonic acid in the air. 
This view was first advanced by Kcehne, of Brussels, in 
1856, and later discussions have strengthened its probabil- 
ity. Part of the oxygen is certainly consumed by weath- 
ering processes, and absorbed e. g., by sulphides and by 
ferro-salts; without this oxidation the actual quantity of 
oxygen in the air would be greater. On the other hand, 
there are in the sedimentary strata many oxidizable com- 
pounds e. g., especially iron sulphides which have prob- 
ably been reduced by the interaction of carbon (by or- 



ganic compounds). A large number of the substances 
which consume oxygen during their decomposition and 
decay have also been produced by the intermediation of 
the coal which had previously been deposited under lib- 
eration of oxygen, so that these substances are, by their 
oxidation, restored to their original state. We may hence 
take it as established that the masses of free oxygen in the 
air and of free carbon in the sedimentary strata approxi- 
mately correspond to each other, and that probably all 
the oxygen of the atmosphere owes its existence to plant 
life. This appears plausible also for another reason. 
We know for certain that there is some free oxygen in the 
atmosphere of the sun, and that hydrogen abounds in 
the sun. The earth's atmosphere may originally have 
been in the same condition. When the earth cooled 
gradually, hydrogen and oxygen combined to water, but 
an excess of hydrogen must have remained. The pri- 
meval atmosphere of the earth may also have contained 
hydrocarbons, as they play an important part in the 
gases of comets. To these gases there were added car- 
bonic acid and water vapor, coming from the interior of 
the earth. Thanks to its chemical inertia, the nitrogen 
of the air may not have undergone much change in the 
course of the ages. An English chemist, Phipson, claims 
to have shown that both higher plants (the corn-bind) 
and lower organisms (various bacteria) can live and 
develop in an atmosphere devoid of oxygen when it 
contains carbonic acid and hydrogen. It is also possible 
that simple forms of vegetable life existed before the air 
contained any oxygen, and that these plants liberated 
the oxygen from the carbonic acid exhaled by the craters. 
This oxygen gradually (possibly under the influence of 
electric discharges) converted the hydrogen and the hydro- 
carbons of the air into water and carbonic acid until those 



elements were consumed. The oxygen remained in the 
air, whose composition gradually approached more the 
actual state. 1 

This oxygen is an essential element for the production 
of animal life. As animal life stands above vegetable life, 
so animal life could only originate at a later stage than 
plant life. Plants require, in addition to suitable tem- 
perature, only carbonic acid and water, and these gases 
will probably be found in the atmospheres of all the 
planets as exhalations of their inner incandescent masses 
which are slowly cooling. The presence of water vapor 
has directly been established, by means of the spectro- 
scope, in the atmospheres of other planets Venus, Jupi- 
ter, and Saturn and indirectly by the observation of a 
snow-cap on Mars. The spectroscope further gives us 
indication of the presence of other gases. There is an 
intense band in the red part of the spectra of Jupiter and 

1 According to the opinion of a colleague of mine, a botanist, the 
results of the experiments of Phipson must be regarded as very doubt- 
ful, and some oxygen would appear to be indispensable for the growth 
of plants. We have to imagine the development somewhat as follows: 
As the earth separated from the solar nebula, its temperature was very 
high at first in its outer portions. At this temperature it was not able 
to retain the lighter gases, like hydrogen and helium, for a long period; 
the heavy gases, like nitrogen and oxygen, remained. The original 
excess of hydrogen and helium disappeared, therefore, before the crust 
of the earth had been formed, and the atmosphere of the earth im- 
mediately after the formation of the crust contained some oxygen, 
besides much nitrogen, carbonic acid, and water vapor. The main 
bulk of the actual atmospheric oxygen would therefore have been 
reduced from carbon dioxide by the intermediation of plants. The 
view that celestial bodies may lose part of their atmosphere is due to 
Johnstone Stoney. The atmospheric gases escape the more rapidly 
the lighter their molecules and the smaller the mass of the celestial 
bodies. On these lines we explain that the smaller celestial bodies 
like the moon and Mercury, have lost almost all their atmosphere, 
while the earth lias only lost hydrogen and helium, which again have 
been retained by the sun. 



Saturn, of wave-length 0.000618 mm. Other new con- 
stituents of unknown nature have been discerned in the 
spectra of Uranus and Neptune. On the other hand, 
there is hardly any, or at any rate only a quite insignifi- 
cant, atmosphere on the moon and on Mercury. This is 
easily understood. The temperature on that side of Mer- 
cury which is turned away from the sun is near absolute 
zero. All the gases of the planetary atmosphere would 
collect and condense there. If, then, Mercury had orig- 
inally an atmosphere, it must have lost it as it lost its 
own rotation, compelling it to turn always the same face 
towards the sun. Similar reasons may account for the 
absence of a lunar atmosphere. If Venus should like- 
wise always turn the same side towards the sun, as many 
astronomers assert, Venus should not have any notable 
atmosphere, nor clouds either. We know, however, that 
this planet is surrounded by a very marked developed 
atmosphere. 1 

And that is the strongest objection to the assumption 
that Venus follows the example of Mercury as regards the 
rotation about its own axis. 

Since, now, warm ages have alternated with glacial 
periods, even after man appeared on the earth, we have 
to ask ourselves : Is it probable that we shall in the com- 
ing geological ages be visited by a new ice period that 
will drive us from our temperate countries into the hotter 
climates of Africa? There does not appear to be much 
ground for such an apprehension. The enormous com- 
bustion of coal by our industrial establishments suffices 
to increase the percentage of carbon dioxide in the air to 
a perceptible degree. Volcanism, whose devastations 

1 That results from the very strong refraction which light undergoes 
in the atmosphere of Venus when this planet is seen in front of the 
sun's edge during the so-called Venus transits. 


Fig. 17. Photograph of the surface of the moon, in the vicinity of 
the crater of Copernicus. Taken at the Yerkes Observatory, Chi- 
cago, U. S. A. Scale: Diameter of moon, 0.55 m.=21.7 in. Owing 
to the absence of an atmosphere and of atmospheric precipitations, 
the precipitous walls of the crater and other elevations do not in- 
dicate any signs of decay 



on Krakatoa (1883) and Martinique (1902) have been 
terrible in late years, appears to be growing more intense. 
It is probable, therefore, that the percentage of carbonic 
acid increases at a rapid rate. Another circumstance 
points in the same direction; that is, that the sea seems 
to withdraw carbonic acid from the air. For the carbonic 
acid percentage above the sea and on islands is on an 
average 10 per cent, less than the above continents. 

If the carbonic acid percentage of the air had kept 
constant for ages, the percentage of the water would 
have found time to get into equilibrium with it; but 
the sea actually absorbs carbonic acid from the air. 
Thus the sea-water must have been in equilibrium with 
an atmosphere which contained less carbonic acid than 
the present atmosphere. Hence the carbonic acid per- 
centage has been increasing of late. 

We often hear lamentations that the coal stored up in 
the earth is wasted by the present generation without 
any thought of the future, and we are terrified by the 
awful destruction of life and property which has followed 
the volcanic eruptions of our days. We may find a kind 
of consolation in the consideration that here, as in every 
other case, there is good mixed with the evil. By the 
influence of the increasing percentage of carbonic acid 
in the atmosphere, we may hope to enjoy ages with more 
equable and better climates, especially as regards the 
colder regions of the earth, ages when the earth will bring 
forth much more abundant crops than at present, for the 
benefit of rapidly propagating mankind. 



THE question has often been discussed in past ages, 
and again in the last century, in how far the position of 
our earth within the solar system may be regarded as 
secure. One might apprehend two things. Either the 
distance of the earth from the sun might increase or 
decrease, or the rotation of the earth about its axis 
might be arrested; and either of these possibilities would 
threaten the continuance of life on the earth. The 
problem of the stability of the solar system has been 
investigated by the astronomers, and their patrons have 
offered high prizes for a solution of the problem. If 
the solar system consisted merely of the sun and the 
earth, the earth's existence would be secure for ages; 
but the other planets exercise a certain, though small, 
influence upon the movements of the earth. That this 
influence can only be of slight importance is due to the 
fact that the total mass of all the planets does not ag- 
gregate more than one-seven-hundred-and-fiftieth of the 
mass of the sun, and, further, to the fact that the planets 
all move in nearly circular orbits around the centre, the 
sun, so that they never approach one another closely. 
The calculations of the astronomers demonstrate that 
the disturbances of the earth's orbit are merely periodi- 
cal, representing long cycles of from 50,000 to 2,000,000 
years. Thus the whole effect is limited to a slight vacil- 



lation of the orbits of the planets about their mean posi- 

So far everything is well and good. But our solar 
system is traversed by other celestial bodies, mostly of 
unknown, but certainly not of circular orbits namely, 
the comets. The fear of a collision with a comet still 
alarmed the thinkers of the past century. Experience 
has, however, taught us that collisions between the 
earth and comets do not lead to any serious con- 
sequence. The earth has several times passed through 
the tails of comets for instance, in 1819 and 1861 
and it was only the calculating astronomer who became 
aware of the fact. Once on such an occasion we have 
thought that we observed a glow like that of an aurora 
in the sky. When the earth was drawing near the 
denser parts of the comet, particles fell on the earth 
in the shape of showers of shooting-stars, without doing 
any appreciable damage. The mass of comets is too 
small perceptibly to disturb the paths of the planets. 

The rotation of the earth about its axis should slowly 
be diminished by the effects of the tides, since they act 
like a brake applied to the surface of the earth. This re- 
tardation is, however, so unimportant that the astronomers 
have not been able to establish it in historical times. The 
slow shrinkage of the earth somewhat counteracts this 
effect. Laplace believed that we were able to deduce, 
from an analysis of the observations of solar eclipses in 
ancient centuries, that the length of the day had not 
altered by more than 0.01 second since the year 729 

We know that the sun, ^accompanied by its planets, 
is moving in space towards the constellation of Hercules 
with a velocity of 20 km. (13 miles) per second, which is 
amazing to our terrestrial conceptions. Possibly the con- 



stituents of our solar system might collide with some other 
unknown celestial body on this journey. But as the celes- 
tial bodies are sparsely distributed, we may hope that many 
billions of years will elapse before such a catastrophe will 
take place. 

In mechanical respects the stability of our system ap- 
peared to be well established. Since the modern theory 
of heat has made its triumphant entry into natural science, 
however, the aspect of matters has changed. We are 
convinced that all life and all motion on the earth can 
be traced back to solar radiation. The tidal motions alone 
make a rather unimportant exception. We have to ask 
ourselves: Will not the store of energy in the sun, which 
goes out, not only to the planets, but to a far greater 
extent into unknown domains of cold space, come to an 
end, and will not that be the end of all the joys and sor- 
rows of earthly existence? The position appears des- 
perate when we consider that only one part in 2300 mill- 
ions of the solar radiation benefits the earth, and perhaps 
ten times as much the whole system, with all its moons. 
The solar radiation is so powerful that every gramme of 
the mass of the sun loses two calories in the course of a 
year. If, therefore, the specific heat of the sun were the 
same as that of water, which in this respect surpasses 
most other substances, the solar temperature would fall 
by 2 Cent. (3.6 F.) every year. As, now, the tempera- 
ture of the sun in its outer portion has been estimated at 
from 6000 to 7000, the sun should have cooled complete- 
ly within historical times. And though the interior of 
the sun most probably has a vastly higher temperature 
than the outer portions which we can observe, we should, 
all the same, have to expect that the solar temperature 
and radiation would noticeably have diminished in his- 
torical times. But all the documents from ancient Baby- 



Ion and Egypt seem to point out that the climate at the 
dawn of historical times was in those countries nearly the 
same as at present, and that, therefore, the sun shone 
over the most ancient representatives of culture in the 
same way as it shines on their descendants now. 

The thesis has frequently been advanced, therefore, that 
the sun has in its heat balance not only an expenditure side, 
but also an almost equally substantial income side. The 
German physician R. Mayer, who has the immortal merit 
of first having given expression to the conception of a 
relation between heat and mechanical work, directed his 
attention also to the household of the sun. He sug- 
gested that swarms of meteorites, rushing into the sun 
with an amazing velocity (of over 600 km. per second), 
would, when stopped in their motion, generate heat at 
the rate of 45 million calories per gramme of meteorites. 
In future ages it would be the turn of the planets to sus- 
tain for some time longer the spark of life in the sun, by 
the sacrifice of their own existences. The sun would there- 
fore, like the god Saturn, have to devour its own children 
in order to continue its existence. Of how little avail 
that would be we learn from the consideration that the 
fall of the earth into the sun would not be able to pro- 
long the heat expenditure of the sun by as many as a hun- 
dred years. By their rush into the sun, almost uniformly 
from all sides, the meteorites would, moreover, long since 
have put a stop to the rotation of the sun about its axis. 
Further, by virtue of the increasing mass and the hence 
augmenting attraction of the sun, the length of our year 
would have had to diminish by about 2.8 seconds per year, 
which is in absolute contradiction to the observations of the 
astronomers. According to Mayer's thesis, a correspond- 
ing number of meteorites would, finally, also have to 
tumble upon the surface of the earth, and (according to 



data which will be furnished in Chapter IV.) they should 
raise the surface temperature to about 800. The thesis 
is therefore misleading. 

We must look for another explanation. It occurred to 
Helmholtz, one of the most eminent investigators in the 
domain of the mechanical theory of heat, that, instead of 
the meteorites, parts of the sun itself might fall towards 
its centre, or, in other words, that the sun was shrinking. 
Owing to the high gravitation of the sun (27.4 times 
greater than on the surface of the earth), the shrinkage 
would liberate a great amount of heat. Helmholtz cal- 
culated that, in order to cover the heat expenditure of 
the sun, a shrinkage of its diameter by 60 m. annually 
would be required. If the sun's diameter should only 
be diminished by one - hundredth of one per cent. a 
change which we should not be able to establish the 
heat loss would be covered for more than 2000 years. 
That seems at first satisfactory. But if we proceed 
with our estimate, we find that if the sun went on 
losing as much heat as at present for seventeen million 
years it would have to contract within this period 
to a quarter of its present volume, and would there- 
fore acquire a density like that of the earth. Long 
before that, however, the radiation from the sun would 
have been decreased so powerfully that the tempera- 
ture on the earth's surface would no longer rise above 
freezing-point. Helmholtz, on this argument, limited 
the further existence of the earth to about six million 
years. That is less satisfactory. But we know noth- 
ing of the future and must be content with possibili- 
ties. Not so, however, if we calculate back with the aid 
of Helmholtz' s theory. According to this theory, and 
according to Helmholtz 's own data, a state like the pres- 
ent cannot have existed for more than ten million years. 


Since, now, geologists have come to the conclusion that 
the petrefactions which we find in the fossil-bearing strata 
of the earth have needed at least a hundred million years 
for their formation, and more probably a thousand million 
years, and since, moreover, the still more ancient forma- 
tions the so-called precambrian strata have been de- 
posited in equally long or still longer periods, we see that 
the theory of Helmholtz is unsatisfactory. 

A somewhat peculiar way out of the dilemma has been 
suggested by a few scientists. We know that one gramme 
of the wonderful element radium emits about 120 calories 
per hour, or in the course of a year, in round numbers, 
a million calories. This radiation seems to continue un- 
impaired for years. If we- now assume that each kilo- 
gramme of the mass of the sun contains only two milli- 
grammes of radium, that amount would be sufficient to 
balance the heat expenditure of the sun for all future 
ages. Without some further auxiliary hypothesis, we / 
can, however, not listen to this suggestion. It prev 
supposes that heat is created out of nothing. Some 
scientists, indeed, believe that radium may absorb a 
radiation, coming from space, in some unknown manner 
and convert it into heat. Before we enter seriously into 
a discussion of this explanation we shall have to answer 
the questions where that radiation comes from and where 
it takes its store of energy. 

We must, therefore, again search for another source of 
heat energy for the sun. Before we can hope to find it, 
we had better study the sun itself a little. 

All scientists are agreed that the sun is of the same 
constitution as the thousands of luminous stars which 
we see in the sky. According to the color of the light 
which they emit, stars are classified as white, yellow, 
and red stars. The differences in their light become 



much more distinct when we examine them spectro 
scopically. In the white stars the helium and hydrogen 
lines predominate decidedly; the helium stars contain, 
in addition, oxygen. Metals are comparatively little rep- 
resented; but they play a main part in the spectra of 
the yellow stars, in which, further, some bands become 
visible. In the spectra of the red stars we notice many 
bands which indicate that chemical compounds are pres- 
ent in the outer portions. Everybody knows that the 
platinum wire or the filament of an incandescent lamp 
which has been heated to incandescence by the electric 
current first shines reddish, then yellow when the current 
is increased, and finally more and more white. At the 
same time the temperature rises. We can estimate the 
temperature from the brightness of the glow. If we 
know the wave-length of the radiations of that color 
which emits the greatest amount of heat in the spectrum 
(it should be a normal spectrum), it is easy to calculate 
the temperature of the star from Wien's law of displace- 
ments. We need only divide 2.89 by the respective 
wave-length expressed in mm. to find the absolute tem- 
perature of the star; by deducting 273 from the result, 
we obtain the temperature in degrees Cent, on the ordi- 
nary scale. For the sun the maximum of heat radiation 
lies near wave-length 0.00055 (in the greenish-yellow 
light), and therefore the absolute temperature of the 
radiating disk of the sun, the so-called photosphere, should 
be 5255 absolute, or nearly 5000 Cent. But our atmos- 
phere weakens the sunlight, and it also causes a displace- 
ment of the maximum radiation in the spectrum. The 
same applies to the sun's own atmosphere, so that we 
have to adopt a higher estimate than 5000 Cent. By 
means of Stefan's law of radiation, the solar temperature 
has been estimated at about 6200, which would corre- 



spond to a wave-length of about 0.00045 mm. This cor- 
rection is therefore significant. About half of it has to 
be ascribed to the influence of the solar atmosphere, the 
other half to the terrestrial atmosphere. A Hungarian 
astronomer, Harkanyi, has determined in the same way 
the temperature of several white stars (Vega and Sirius), 
and found it to be about 1000 higher than that of the 
sun, while the red star Betelgeuse, the most prominent 
star in Orion, would have a temperature by 2500 lower 
than that of the sun. 

It must expressly be stated that in making these esti- 
mates we understand by the temperature of the star in 
this case the temperature of a radiating body which emits 
the same light as that which reaches us from the star. 
But the stellar light undergoes important changes on its 
way to us. We learn from observing new stars t that a 
star may be surrounded by a cloud of cosmical dust which 
sifts the blue rays out and permits the red ones to pass. 
The star then shines with a less brilliantly white light than 
in the absence of the cloud. The consequence is that we 
estimate the temperature lower than it really is. In the 
red stars bands have been noticed, indicating, as we have 
already said, the presence of chemical compounds. The 
most interesting of these are the compounds of cyanogen 
and of carbon, probably with hydrogen, which appear to 
resemble those observed by Swan in the spectrum of gas 
flames and which were named after him. It was formerly 
thought that the presence of these compounds implied 
lower temperature. But we shall see that this conclusion 
is not firmly established. Hale has found during eclipses 
of the sun that exactly the same compounds occur im- 
mediately above the luminous clouds of the sun. They 
are probably more numerous below the clouds, where the 
temperature is no doubt higher, than above them. 

6 71 


However that may be, we have reason to assume that 
the now yellow sun was once a white star like the brill- 
iant Sirius, that it has slowly cooled down to its present 
appearance, and that it will some day shine with the red- 
dish light of Betelgeuse. The sun will then only radiate 
a seventh of the heat which it emits now, and it is very 
likely that the earth will have been transformed into a 
glacial desert long before that time. 

It has already been pointed out that the atmospheres 
of both the sun and of the earth produce a strong ab- 
sorption of the solar rays, and especially of the blue and 
white rays. It is for this reason that the light of the sun 
appears more red in the evening than at noon, because 
in the former case it has to pass through a thicker layer 
of air, which absorbs the blue rays. For the same reason 
the limb of the sun appears more red in spectroscopic 
examinations than the centre of the sun. This weaken- 
ing of the sun's light is due to the fine dust pervading 
the atmospheres of the earth and the un. When the 
products of strong volcanic eruptions, like the eruptions 
of Krakatoa in 1883 and of Mont Pelee in 1902, filled the 
atmosphere with a fine volcanic dust, the sun appeared 
distinctly red when standing low in the horizon. It was 
this dust that caused the red glow. 

When we examine an image of the sun which has 
been thrown on a screen by the aid of a lens or a system 
of lenses, we notice on the sun's disk a mottling of charac- 
teristic darker spots. These spots struck the attention 
of Galileo, and they were discovered almost simultane- 
ously by him, by Fabricius, and by Scheiner (1610-1611). 
These spots have since been the most diligently studied 
features of the sun. We carefully determine their number 
and sizes, and combine these two data to make the 
so-called sun-spot numbers. These numbers change 



from year to year in a rather irregular way, the period 
amounting on an average to 11.1 years. The spots appear 
in two belts on the sun, and they glide over the disk in 
the course of thirteen or fourteen days. Sometimes they 
reappear after another thirteen or fourteen days. It is 
therefore believed that they lie comparatively quiet on the 
surface of the sun, and that the sun rotates about its own 
axis in about twenty-seven days, so that after that period 
the same points are again opposite the earth. This is 
the so-called synodical period. The great interest which 
attaches to the study of these features lies in the fact that 
simultaneously with these spots several other phenomena 
seem to vary which attain their maxima at the same time. 
Such are, in the first instance, the polar lights and the 
magnetic variations, and, to a lesser degree, the cirrus 
clouds and temperature changes, as well as several other 
meteorological phenomena (compare Chapter V.). 

About the sun-spots we notice the so-called faculse 
portions which are much brighter than their surround- 
ings. When we carefully examine a strongly magnified 
image of the sun, we find that it has a granulated ap- 
pearance (Fig. 18). Langley compares the disk to a 
grayish-white cloth almost hidden by flakes of snow. 
The less bright portions are designated "pores," the 
brighter portions "granules." It is generally assumed 
that the granules correspond to clouds which rise like 
the clouds of our atmosphere on the top of ascending 
convection currents. But while the terrestrial clouds are 
formed of drops of rain or of crystals of ice, the granules 
consist probably of soot that is to say, condensed car- 
bon and of drops of metals, iron, and others. The 
smallest granule which we are able to discern has a 
diameter of about 200 km. (130 miles). 

The faculse are formed by very large accumulations of 



Fig. 18. Sun-spot group and granulation of the sun. (Photographed 
at the Meudon Observatory, near Paris, April 1, 1884) 

clouds which are carried up by strong ascending currents 
and spread over large areas, as in our cyclones. The 
spots correspond to descending masses of gas with rising 
temperatures, which are therefore "dry" and do not 
carry any clouds, as in terrestrial anticyclones. Through 



these holes in the walls of solar clouds we peep a little 
farther into the gigantic masses of gas, and we obtain an 
idea of the state of affairs in the deeper strata of the sun. 
The depth of the wall of cloud is, of course, not large 
compared to the radius of the sun. 

The study of the spectra affords us the best insight 
into the nature of the different parts of the sun. The 
spectra teach us not only the constituents of these parts, 
but also the velocities with which they move. We have 
learned in this way that, lying above the luminous clouds 
of the sun which are radiating to us, there are great masses 
of gas containing most of our terrestrial elements. We 
distinguish particularly in them iron, magnesium, cal- 
cium, sodium, helium, and hydrogen. The two last- 
mentioned constituents, being the least dense, are found 
particularly in the outermost strata of the atmosphere. 
The solar atmosphere becomes visible when, during an 
eclipse of the sun, the disk of the moon has proceeded so 
far as to cover the intensely luminous clouds in the so- 
called photosphere. Owing to its strong percentage of 

Fig. 19. Part of the solar spectrum of January 3, 1872. After Langley. 
The bright horizontal bands are due to prominences. . In the middle 
(at 208) the hydrogen line F, strongly distorted by violent agitation 



hydrogen, the gaseous atmosphere generally shines in the 
purple hue which is characteristic of this element. This 
stratum of gas is also called the chromosphere (from the 
Greek word %p<w//,a, meaning color). Its thickness is 
estimated at from 7000 to 9000 km. (5000 to 6000 miles). 

Fig. 20. Metallic promi- 
nences in vortex motion 

The white spot marks the Fig. 21. Fountain-like metallic 
size of the earth prominences 

From it rise rays of fire over the surrounding surface like 
blades of grass on meadows, to which their appearance 
has been likened. 

When these flames rise still higher, to about 15,000 
km. (9300 miles) or more, they are called protuberances 
or prominences. Their number as well as their altitude 
grow with the number of sun - spots. They are dis- 
tinguished as metallic and as quiet prominences. The 
former are characterized by particularly violent mo- 
tion, as will become apparent from Figs. 20 and 21, 
and they contain large amounts of metallic vapor. 
They appear only within the belt of sun-spots which 
are most pronounced at a distance of about 20 from 
the solar equator. Their movements are so violent that 
they often traverse several hundreds of kilometres in a 
second. The Hungarian Fenyi observed, indeed, on July 
15, 1895, a prominence whose greatest velocity in the line 



of sight, measured spectroscopically, amounted to 862 km. 
(536 miles) , and whose maximum velocity at right angles 
to this direction was 840 km. per second. These colossal 
velocities distinguish the highest parts, while the lower 
portions, which are the most dense and which contain 
most metallic vapor, are less mobile, as might be expected. 
Their altitude above the sun's surface may reach ex- 
ceedingly high figures, and this applies also to the quiet 
prominences. The above-mentioned prominence of July 
15, 1895, reached a height of 500,000 km., and Langley 
observed, on October 7, 1880, one at an altitude of 560,000 
km., whose tip, therefore, nearly attained an elevation 
equal to that of a radius of the sun, 690,000 km. above 
the limb of the sun's photosphere. The mean altitude 
of these prominences is 40,000 km. After their dis- 
covery by Lector Vassenius, of Gotheborg, in 1733, they 
could only be studied during total solar eclipses, until 

Fig;. 23. Quiet prominences, shape 
Fis;. 22. Quiet prominences of of a tree. , The white spot indi- 

smoke-column type cates the size of the earth 

Lockyer and Janssen taught us, in the year 1868, how 
to observe them in full sunlight by means of the spec- 
The quiet prominences consist almost exclusively of 



hydrogen and helium; sometimes they contain also 
traces of metallic gases. They resemble clouds floating 
quietly in the solar atmosphere, or masses of smoke com- 


Fig. 24. Diagram illustrating the differences in the spectra of sun- 
spots and of the photosphere. Some lines in the spot spectrum are 
stronger, others fainter, than in the photosphere spectrum. In the 
central portion, two reversals; to the right, two bands. After Mitchell 

ing from a chimney. They may appear anywhere on the 
sun, and their stability is so great that they have some- 
times been watched during a complete solar rotation (for 

Fig. 25. Spectrum of a sun-spot, the central band between the two 
portions of the photosphere spectrum. The spot spectrum is bor- 
dered with the half -shadows of the edge of the spot. After Mitchell 



about forty days) ; this is possible only when they occur 
in the neighborhood of the poles, where they always re- 
main visible outside the sun's limb. Figs. 22 and 23 
show several such prominences according to Young. 
Sometimes the matter of the prominences seems to fall 

A 7 




Fig. 26. The great sun-spot of October 9, 1903. Taken with the 
photo-heliograph of Greenwich in the usual manner. The spot is 
shown at mean level of the calcium faculse. The two following 
photographs show a lower-level and a higher-level section through 
the calcium faculae 

back upon the surface of the sun between the smaller 
flames of fire which we have likened to blades of grass 
(Fig. 21). In most cases, however, the prominences ap- 
pear slowly to dissolve. When their brilliant glow fades 
owing to their intense radiation, they can no longer 



be observed. The quiet prominences, which seem to 
float at heights of about 50,000 km. and at still greater 
heights, must there be almost in a vacuum. Their par- 
ticles cannot be supported by any surrounding gases, 
after the manner of the drops of water in terrestrial 
clouds. In order that they may remain floating they 
must be pushed away from the sun by a peculiar force 
the radiation pressure (see Chapter IV.). 

The facula3 can be studied in the same way as the 
prominences, and of late Deslandres and Hale have used 
for this purpose a special instrument, the heliograph 
(compare Figs. 26 to 29). When the faculse approach 
the limb of the sun they appear particularly brilliant by 
comparison with their surroundings. That seems to in- 
dicate that they are lying at a great altitude., and that 

Fig. 27. The great sun-spot of October 9, 1903. Photograph of the 
low-level calcium faculae with the aid of the light of the calcium 
line H. The spot is not obscured by the faculan at least, not so 
much as in the following illustrations 



their light is hence not weakened by the superposed hazy 
stratum. When they reach the sun's limb they appear 
to us like raised portions of the photosphere. The clouds 

Fig. 28. The great sun-spot of October 9, 1903. Photograph of the 
higher-level calcium faculse, taken with the light of the central portion 
of the line H (calcium). The higher-level faculse hide the spot, in- 
dicating that the faculaB spread considerably during their ascent 

which form these faculse are carried upward by powerful 
ascending streams of gas whose expansion is due to the 
diminution of the gaseous pressure. 

Sun-spots display many peculiarities in their spectra 
(Figs. 24 and 25). Very prominent is always the helium 
line ; prominent likewise the dark sodium lines, which are 
markedly widened and which show in their middle portions 
a bright line the so-called reversal of lines (Fig. 24) . This 
occurrence indicates that the metal is lying in a deeper 
stratum. In the red portion of the spectrum we find 



bands, just as in the spectra of the red stars. These bands, 
which appear to be resolved into crowds of lines by the 
aid of powerful instruments, indicate the presence of 
chemical compounds. Since the spot is comparatively 
of feeble intensity, its spectrum appears superposed 
like a less bright ribbon upon the background of the 
spectrum of the more luminous photosphere. The violet 
end of the sun-spot spectrum is particularly weakened. 
Although the spot has the appearance of a pit in the 

Fig. 29. The great sun-spot of October 9, 1903. Photograph of the 
hydrogen faculse, taken with the light of the spectral line F (hydro- 
gen). Only the darkest portions of the spot are visible. The other 
portions are obscured by masses of the hydrogen, which were evidently 
in a restless state 

photosphere, and when on the sun's limb makes it look 
as if a piece had been cut out of the edge, it yet 
does not appear darker than, the sun's edge. That points 



Fig. 30. Photograph of the solar corona of 1900, (After 
Langley and Abbot.) Illustrating the appearance of the 
corona in years of minimum sun-spot frequency 

to the conclusion that the light emitted by the spot 
emanates chiefly from its upper, cold portions. 

The light coming from the deeper portions is distinctly 
absorbed to a large degree by the higher-lying strata. 
The sun-spots also appear to become narrower in their 
lower parts, owing to the compression of the gases at 
greater depths, and one may regard their funnel-shaped 
cloud-walls as " half-shadows," which appear darker than 
the surroundings, but brighter than the so-called core of 
the spot. The weakening of the violet end of the spec- 
trum is probably due to the presence of fine particles of 



dust in the solar gases, just as they cause* the corre- 
sponding weakening of the violet end of the spectrum of 
the sun's limb. The bands in the red parts of the sun- 
spot spectrum may originate from the deeper portions 
of the spot, because all the higher parts of the solar 

Fig. 31. Photograph of the solar corona of 1870. (After Davis.) 
The year 1870 was one of maximum sun-spot frequency 

atmosphere yield simple, sharp lines. The bands suggest 
that chemical compounds can exist at the higher press- 
ure of the inner portions of the sun, and that these com- 



pounds are decomposed in the outer parts of the sun, to 
give the line spectra of chemical elements. 

The enigmatical corona lies farther out in the atmos- 
phere of the sun. It consists of streamers which may 
extend beyond the disk of the sun to the length of several 
solar diameters. The corona can only be observed at 
total eclipses of the sun. Figs. 30 to 32 illustrate the ap- 
pearance of this very peculiar phenomenon. 

When the number of sun-spots is small, the corona 
streamers extend like huge brooms from the equatorial 

Fig. 32. Photograph of the solar corona of 1898. (After Maunder.) 
1898 was a year of average solar activity 

parts, and the feebler rays of the corona near the solar 
poles are then bent downward to the equator, just like' 
the lines of force about the poles of a magnet (Fig. 30). 
We suppose, for this reason, that the sun acts like a 
strong magnet, whose poles are situated near the geo- 
graphical poles of the sun. In years which are richer in 
sun-spots the distribution of the streamers of the corona 
is more uniform. At moderate sun-spot frequency, large 
numbers of rays seem to emanate from the neighborhood 



of the maximum belt of sun-spots, so that the corona 
often assumes a quadrangular shape (compare Fig. 32). 

These remarks hold for the "outer corona," while the 
inner portion, the so-called "inner corona," shines in a 
more uniform light. The spectroscopic examination 
demonstrates that the light consists mainly of hydrogen 
gas and of an unknown gas designated coronium, which 
particularly seems to occur in the higher parts of the 
inner corona. The outer streamers of the corona, on the 
contrary, yield a continuous spectrum which shows that 
the light is radiated by solid or liquid particles. In the 
spectrum of the coronal rays at an extreme distance from 
the disk, astronomers have sometimes fancied that they 
discerned dark lines on a bright ground, just as in the 
spectrum of the photosphere. It has been assumed that 
this light is reflected sunlight, originating from small solid 
or liquid particles of the outer corona. It must be re- 
flected, because it is partly polarized. The radiating dis- 
position of the outer corona indicates the action of a 
force, the radiation pressure, which drives the smaller 
particles away from the centre of the sun. 

As regards the temperature of the sun, we have already 
seen that the two methods applied for its determination 
have yielded somewhat unequal results. From the in- 
tensity of the radiation, Christiansen, and afterwards 
Warburg, calculated a temperature of about 6000 Cent. 
Wilson and Gray found for the centre of the sun 6200, 
which they afterwards corrected into 8000. Owing to 
the absorption of light by the terrestrial and the solar 
atmospheres, we always find too low values. That ap- 
plies, to a still greater extent, to any estimate based upon 
the determination of that wave-length for which the heat 
emission from the solar spectrum is maximum. Lc 
Chatelier compared the intensity of sunlight filtered 



through red glass with the intensities of light from sev- 
eral terrestrial sources of fairly well-known temperatures 
treated in the same way. These estimates yielded to 
him a solar temperature of 7600 Cent. Most scientists 
reckon with an absolute temperature of 6500, corre- 
sponding to about 6200 Celsius. That is what is known 
as the "effective temperature" of the sun. If the solar 
rays were not partially absorbed, this temperature would 
correspond to that of the clouds of the photosphere. Since 
red light is little absorbed comparatively, Le Chatelier's 
value of 7600, and the almost equal value of Wilson and 
Gray of 8000, should approximately represent the average 
temperature of the outer portions of the clouds of the 
photosphere. The higher temperature of the faculse is 
evident from their greater light intensity, which, how- 
ever, may partly be due to their greater height. Carring- 
ton and Hodgson saw, on September 1, 1859, two faculse 
break out from the edge of a sun-spot. Their splendor 
was five or six times greater than that of the surrounding 
parts of the photosphere. That would correspond to 
a temperature of about 10,000 or 12,000 Cent: The 
deeper parts of the sun which broke out on these occa- 
sions evidently have a higher temperature, and this is 
not unnatural, since the sun is losing heat by radiation 
from its outer portions. 

We know that the temperature of our atmosphere de- 
creases with greater heights. The movements of the air 
are concerned in this change. A sinking mass of air is 
compressed by the increased pressure to which it is being 
exposed, and its temperature rises, therefore, just as the 
temperature rises in a pneumatic gas-lighter when the 
piston is pressed down. If the air were dry and in strong 
vertical motion, its temperature would change by 10 Cent. 
(18 F.) per km. If it stood still, it would assume an al- 
7 87 


most uniform temperature; that is to say, there would 
be no lowering of the temperature as we proceed upward. 
The actual value lies between the two extremes. As the 
gravitation in the photosphere of the sun is 27.4 times 
greater than on the surface of the earth, we can deduce 
that, if the air on the sun were as dense as on the earth, 
the temperature on the sun would vary 27.4 times as 
much as on the earth with the increasing height that 
is to say, by 270 degrees per kilometre, provided its atmos- 
phere were in violent agitation. Now, the outer portions 
of the solar atmosphere are, indeed, in violent motion, so 
that this latter assumption seems to be justified. But 
this part consists essentially of hydrogen, which is 29 
times lighter than the air. We must, therefore, reduce 
the value at which we arrive to one-twenty-ninth. As a 
result, the final temperature gradient per kilometre would 
only be 9 Cent. (16.2 F.). But the radiation is extreme- 
ly powerful on the sun, and it tends to equalize the con- 
ditions. Nine degrees per kilometre is therefore, without 
doubt, too high a value. Further, in the interior of the 
sun the gases are much heavier. At a small depth, how- 
ever, they will be so strongly compressed by the upper 
strata that their further compressibility will be limited, 
and the calculation which we have just made loses its 
validity. Yet, in any case, the temperature of the sun 
must increase as we penetrate nearer to its centre. If 
we accept a temperature gradient per kilometre of the 
value above indicated, 9 it is three times greater in the 
solid earth-crust we should obtain for the centre of the 
sun a temperature of more than six million degrees. 

All substances melt and evaporate as their tempera- 
ture is raised. If the temperature exceeds a certain 
limit, the "critical temperature," the substance can no 
longer be condensed to a liquid, however high the press- 


ure may be pushed, and the substance will only exist 
as a gas. If we start from 273 as absolute zero, 
this critical temperature is nearly one and a half times 
as high as the ebullition temperature of the sub- 
stance under atmospheric pressure. So far as our ex- 
perience goes, it does not appear probable that the 
critical temperature of any substance could be higher 
than 10,000 or 12,000 Cent., the highest values which 
we have calculated for the temperature of the faculae. 
The inner portions of the sun must hence be gaseous, 
and the whole sun be a strongly compressed mass of gas 
of extremely high temperature, which, owing to the high 
pressure, is at a density 1.4 times as great as that of 
water, and which in many respects, therefore, will re- 
semble a liquid. It must, for instance, be extremely 
viscid, and that accounts for the relatively great stability 
of the sun-spots (one sun-spot held out for a year and a 
half in 1840 and 1841). The sun would thus have to be 
regarded as a sphere of gas, in the outer portions of which 
a certain amount of condensations of cloud character 
have taken place, owing to radiation and to the out- 
ward movements of the gaseous masses. The pressure 
in the photosphere that is, in those parts in which 
these clouds are floating has been averaged at five or 
six atmospheres, a figure which, considering the very 
high gravitation, would suggest a layer of superposed 
gas above it corresponding to not more than a fifth 
of our terrestrial atmosphere. At an approximately cor- 
responding height, 11,500 m. (38,000 ft.), there are float- 
ing in the terrestrial atmosphere the highest cirrus clouds, 
to which the clouds of the photosphere may in many 
respects be compared. 

We turn back to the unanswered question whence the 
sun takes the compensation for the heat which it con- 



stantly radiates into space. The most powerful source 
of heat known to us is that of chemical reactions. The 
most familiar reaction of daily life is the combustion of 
coal. By burning one gramme of carbon we obtain 
8000 calories. If the sun consisted of pure carbon, its 
energy would not hold out more than 4000 years. It is 
not to be wondered at, therefore, that most scientists 
soon abandoned the hope of solving the problem in this 
way. The French astronomer Faye attempted to explain 
the replenishment of the losses of heat by radiation 
from the sun by arguments in which he resorted to 
the heat of a combination of the constituents of the 
sun. He said: "So high a temperature must prevail in 
the interior of the sun that everything there will be de- 
composed into its elementary constituents. When the 
atoms afterwards penetrate into the outer layers, they 
are again united, and they liberate heat." Faye thus 
imagined that new masses of elements would constantly 
rise from the interior of the sun and would be reunited 
in chemical combination on the surface. But if new 
masses are to penetrate upward to the surface, those 
which were at first above must go back to the centre of 
the sun, in order to be re-decomposed by the great heat 
there; and this re-decomposition would consume just as 
much heat as was gained by the rising of the same masses 
to the surface. This convection can therefore only help 
to transport the store of heat from the interior to the 
surface. The total amount of heat stored in the sun 
would in this way, supposing the mean temperature to 
be six million degrees, be able to cover the heat ex- 
penditure for about three million years. 

We have, moreover, seen that the highest strata 
of the sun are distinguished by line spectra, suggestive 
of simple chemical compounds, while at greater depth 



in the sun-spots chemical combinations occur which 
are characterized by band spectra. It is quite incorrect 
to assert that high temperatures must necessarily de- 
compose all chemical compounds into their elements. 
The mechanical theory of heat teaches us only that at 
rising temperatures products are formed whose formation 
goes hand in hand with an absorption of heat. Thus, at 
a high temperature, ozone is formed from oxygen, al- 
though ozone is more complex in composition than 
oxygen, and by this reaction 750 calories are consumed 
when one gramme of oxygen is transformed into one 
gramme of ozone. We likewise know that in the electric 
arc, at a temperature of about 3000, a compound is 
formed under consumption of heat by the oxygen and 
nitrogen of the atmosphere. A new method for the tech- 
nical preparation of nitric acid from the nitrogen of the 
air is based upon this reaction. Again, the well-known 
compounds benzene and acetylene are formed from their 
elements, carbon and hydrogen, under absorption of 
heat. All these bodies can only be synthetized from their 
elementary constituents at high temperatures. We fur- 
ther know from experience that the higher the temper- 
ature at which a reaction takes place, the greater, in 
general, the amount of heat which it absorbs. 

A similar law applies to the influence of pressure. 
When the pressure is increased, such processes will be 
favored as will yield products of a smaller volume. If 
we imagine that a mass of gas rushes down from a higher 
stratum of the sun into the depths of the sun's in- 
terior, as gases do in sun-spots, complex compounds will 
be produced by virtue of the increased pressure. This 
pressure must increase at an immense rate towards the 
interior of the sun, by about 3500 atmospheres per kilo- 
metre. The gases which dissociate into atoms at the lower 



pressures and the higher temperatures of the extreme 
solar strata above the photosphere clouds enter into 
chemical combination in the depths of the spots, as we 
learn from spectroscopic examination. Owing to their 
high temperatures, these compounds absorb enormous 
quantities of heat in their building up, and these quanti- 
ties of heat are to those which are concerned in the chemi- 
cal processes of the earth in the same ratio as the tem- 
perature of the sun is to that at which the chemical 
reactions are proceeding on the earth. As these gases 
penetrate farther into the sun, temperature and pressure 
are still more and more increased, and there will result 
products more and more abounding in energy and con- 
centration. We may, therefore, imagine the interior of the 
sun charged with compounds which, brought to the sur- 
face of the sun, would dissociate under an enormous evolu- 
tion of heat and an enormous increase of volume. These 
compounds have to be regarded as the most powerful 
blasting agents, by comparison with which dynamite and 
gun-cotton would appear like toys. In confirmation of 
this view, we observe that gases when penetrating into 
the photosphere clouds are able to eject prominences at 
a stupendous velocity, attaining several hundred kilo- 
metres per second. This velocity surpasses that of the 
swiftest rifle-bullet about a thousandfold. We may hence 
ascribe to the explosives which are confined in the in- 
terior of the sun energies which must be a million times 
greater than the energy of our blasting agents. (For the 
energy increases with the square of the velocity.) And 
yet these solar blasting agents have already given up 
a large part of their energy during their passage from the 
sun's interior. It thus becomes conceivable that the 
solar energy instead of holding out for 4000 years, as 

it would if it depended upon the combustion of a solar 



sphere made out of carbon will last for something like 
four thousand million years. Perhaps we may further 
extend this period to several billions. 

That there are such energetic compounds we have 
learned from the discovery of the heat evolution of radium. 
According to Rutherford, radmn^is decomposed by one- 
half in the space of about W&& years. In this decom- 
position a quantity of about a million calories is evolved 
per gramme and per year, and we thus find that the de- 
composition of radium into its final products is accom- 
panied by a heat evolution of about two thousand millions 
of calories per gramme about a quarter of a million 
times more heat than the combustion of one gramme of 
carbon would yield. 

In chemical respects as well, then, the earth is a dwarf 
compared to the sun, and we have every reason to pre- 
sume that the chemical energy of the sun will be sufficient 
to sustain the solar heat during many thousand millions 
and possibly billions of years to come. 



NEXT to simple measuring and simple calculations, 
astronomy appears to be the most ancient science. Yet, 
though man has worshipped the sun from the most remote 
ages, it was not fully comprehended before the middle 
of the past century that the sun is the source of all 
life and of all motion. Part of the veneration for the 
sun was transferred to the moon, with its mild light, and 
to the smaller celestial lights. It did not escape notice 
that their positions in the sky were always changing 
simultaneously with the annual variations in the weather, 
and all human undertakings depended upon the weather 
and the seasons. The moon and the stars were worship- 
ped we know now, without any justification whatever 
as ruling over the weather, and consequently over man's 
fate. 1 Before anything was undertaken people attempt- 
ed first to assure themselves of the favorable aspect of 
the constellations, and since the most remote ages as- 
trologers have exercised a vast influence over the igno- 
rant and superstitious multitude. 

In spite of the vehement enunciation of Giordano 

1 The moon strongly, and more than any other agent, influences 
the tides. Apart from this effect the position of the moon has only 
a feeble influence upon the air pressure and upon atmospheric elec- 
tricity and terrestrial magnetism. The influence of the stars is im- 



Bruno (1548-1600), this superstition was still deeply 
rooted when Newton succeeded in proving, in 1686, 
that the movements of the so-called wandering stars, 
or planets, and of their moons could be calculated 
with the aid of one very simple law: that all these 
celestial bodies are attracted by the sun or by their 
respective central bodies with a force which is pro- 
portional to their own mass and to the mass of the 
central body and inversely proportional to the square 
of their distance from that central body. Newton's 
contemporary, Halley, applied the law of gravitation 
also to the mysterious comets, and calculating astron- 
omy has since been based upon this, its firmest law, 
to which there has not been found any exception. The 
world was thus at once rid of the paralyzing superstition 
which exacted belief in a mysterious ruling of the stars. 
The contemporaries of Newton, as well as their descend- 
ants, have rightly valued this discovery more highly 
than any other scientific triumph of this hero's. Ac- 
cording to Newton's law, all material bodies would tend 
to become more and more concentrated and united, and 
the development of the universe would result in the suck- 
ing up of the smaller celestial bodies the meteorites, for 
instance by the larger bodies. 

It must, however, be remarked that Newton's great 
precursor, Kepler, observed in 1618 that the matter of 
the comets is repelled by the sun. Like Newton, he 
believed in the corpuscular theory of light. The sun and 
all other luminous bodies radiated light, they thought, be- 
cause they ejected minute corpuscles of light matter in 
all directions. If, now, these small corpuscles hit against 
the dust particles in the comets' tails, the dust particles 
would be carried away with them, and their repulsion by 

the sun would become intelligible. It is characteristic 



that Newton would not admit this explanation of Kepler's, 
although he shared Kepler's opinion on the nature of light. 
According to Newton, the deviation of the tails of com- 
ets from his law of general attraction was only apparent. 
The tails of comets, he argued, behaved like the columns 
of smoke rising from a chimney, which, although the gases 
of combustion are attracted by the earth, yet ascend 
because they are lighter than the surrounding air. This 
view, which has been characterized by Newcomb as no 
longer to be seriously taken into consideration, demon- 
strates the strong tendency of Newton to explain every- 
thing with the aid of his law. 

The astronomers followed faithfully in the footsteps 
of their inimitable master, Newton, and they brushed 
aside every phenomenon which would not fit into his 
system. An exception was made by the famous Euler, 
who, in 1746, expressed the opinion that the waves of 
light exerted a pressure upon the body upon which they 
fell. This opinion, however, could not prevail against 
the criticisms with which others, and especially De Mairan, 
assailed it. That Euler was right, however, was proved 
by Maxwell's great theoretical treatise on the nature of 
electricity (1873). He showed that rays of heat and 
the same applies, as Bartoli established in 1876, to 
radiations of any kind must exercise a pressure just as 
great as the amount of energy contained in a unit vol- 
ume, by virtue of their radiation. Maxwell calculated 
the magnitude of this pressure, and he found it so small 
that it could hardly have been demonstrated with the 
experimental means then at our disposal. But this dem- 
onstration has since been furnished, with the aid of 
measurements obtained in a vacuum, by the Russian 
Lebedeff and by the Americans Nichols and Hull (1900, 
1901). They have found that this pressure, the so-called 



radiation pressure, is exactly as great as Maxwell pre- 

In spite of Maxwell's great authority, astronomers quite 
overlooked this important law of his. Lebedeff, indeed, 
tried in 1892 to apply it to the tails of comets, which he 
regarded as gaseous; but the law is not applicable in this 
case. As late as the year 1900, shortly before Lebedeff 
was able to publish his experimental verification of this 
law, I attempted to prove its vast importance for the ex- 
planation of several celestial phenomena. The magni- 
tude of the radiation pressure of the solar atmosphere 
must be equivalent to 2.75 milligrammes if the rays strike 
vertically against a black body one square centimetre in 
area. I also calculated the size of a spherule of the same 
specific gravity as water, such that the radiation pressure 
to which it would be exposed in the vicinity of the sun 
would balance the attraction by the sun. It resulted 
that equilibrium would be established if the diameter of 
the sphere were 0.0015 mm. A correction supplied by 
Schwarzschild showed that the calculation was only valid 
wlien the sphere completely reflects all the rays which fall 
upon it. If the diameter of the spherule be still smaller, 
the radiation pressure will prevail over the attraction, and 
such a sphere would be repelled by the sun. Owing to 
the refraction of light, this will, according to Schwarz- 
schild, further necessitate that the circumference of the 
spherule should be greater than 0.3 time the wave-length 
of the incident rays. When the sphere becomes still small- 
er, gravitation will once more predominate. But spherules 
whose sizes are intermediate between these two limits will 
be repelled. It results, therefore, that molecules, which 
have far smaller dimensions than those mentioned, will 
not be repelled by the radiation pressure, and that 
therefore Maxwell's law does not hold for gases. When 



the circumference of the spherule becomes exactly equal 
to the wave-length of the radiation, the radiation press- 
ure will act at its maximum, and it will then surpass 
gravity not less than nineteen times. These calculations 
apply to all spheres, totally reflecting the light, of a spe- 
cific gravity like water, and to a radiation and attraction 
corresponding to that of the sun. Since the sunlight is 
not homogeneous, the maximum effect will somewhat be 
diminished, and it is nearly equal to ten times the grav- 
ity for spheres of a diameter of about 0.00016 mm. 1 

Before we had recourse to the radiation pressure for 
the explanation of the repulsion phenomena such as have 
been observed in the tails of comets, it was generally be- 
lieved with Zollner that the repulsion was due to electrical 
forces. Electricity undoubtedly plays an important part 
in these phenomena, as we shall see. The way in which 
it acts in these instances was explained by a discovery of 
C. T. R. Wilson in 1899. Gases can in various ways be 
transformed into conductors of electricity which as a rule 
they do not conduct. The conducting gases are said to 
be ionized that is to say, they contain free ions, minute 
particles charged with positive or negative electricity. 
Gases can be ionized, among other ways, by being ra- 
diated upon with Rontgen rays, kathode rays, or ultra- 
violet light, as well as by strong heat. Since the light 
of the sun contains a great many ultra-violet radia- 
tions, it is indisputable that the masses of gases in 
the neighborhood of the sun (e.g., probably in comets 
when they come near the sun) will partly be ionized, 

1 One centimetre of water contains 470 billions of these spheres. 
Such a little drop of water, again, contains 96 millions of molecules, 
and there are probably organisms which are smaller than these 
drops. Compare the experiments with ultra-microscopic organisms 
by E. Raehlmann, N. Gaidukow, and others. 



and will contain both positive and negative ions. Ion- 
ized gases are endowed with the remarkable capability 
of condensing vapors upon themselves. Wilson showed 
that this property is possessed to a higher degree by the 
negative ions than by the positive ions (in the condensa- 
tion of water vapor). If there are, therefore, water 
vapors in the neighborhood of the sun which can be con- 
densed by cooling, drops of water will, in the first instance, 
be condensed upon the negative ions. When these drops 
are afterwards repelled by the radiation pressure, or when 
they sink, owing to gravity, as drops of rain sink in the 
terrestrial atmosphere, they will carry with them the 
charge of the negative ions, while the corresponding posi- 
tive charge will remain behind in the gas or in the air. 
In this way the negative and positive charges will become 
separated from each other, and electric discharges may 
ensue if sufficiently large quantities of opposite electricity 
have been accumulated. By reason of these discharges 
the gases will become luminescent, although their tem- 
perature may be very low. Stark has even shown that 
low temperatures are favorable for the display of a strong 
luminosity in electric discharges. 

We have stated that Kepler, as early as the beginning 
of the seventeenth century, came to the conclusion that 
the tails of comets were repelled by the sun. Newton 
indicated how we might, from the shape of the comets' 
tails, calculate their velocity. The best way, however, 
is to determine this velocity by direct observation. The 
comets' tails are not so uniform in appearance as they 
are generally represented in illustrations, but they often 
contain several luminous nuclei (Fig. 33), whose motions 
can be directly ascertained. 

From a study of the movements of comets' tails, Olbers 
concluded, about the beginning of the last century, that the 



repulsion of the comets' tails by the sun is inversely pro- 
portional to the square of their distance tMat is to say, 
that the force of the repulsion is subject to the same law 
as the force of gravitation. We can, therefore, express 

Fig. 33. Photograph of Roerdanrs comet (1893 II.), suggesting 
several strong nuclei in the tail 

the repulsion effect in unfts of solar gravitation, and this 
has generally been done. That the radiation pressure 
will in the same manner change with the distance is only 

natural. For the radiation against the same surface is 



Fig. 34. Photograph of Swift's comet (1892 I.) 

also inversely proportional to the square of the distance 
from the radiating body, the sun. 

In the latter part of the past century the Russian as- 
tronomer Bredichin conducted a great many measure- 
ments on the magnitude of the forces with which comets' 
tails are repelled by the sun. He considered himself, on 
the strength of these measurements, justified in dividing 
comets' tails into three classes. In the first class the 
repulsion was 19 times stronger than gravitation; in the 
second class, from 3.2 to 1.5 times stronger; and in the 
third class, from 1.3 to 1 times stronger. Still higher 

values have, however, been deduced -for several comets. 



Thus Hussey found for the comet of 1893 (Roerdam's 
comet, 1893 II. , Fig. 33) a repulsion 37 times as strong 
as gravitation; and Swift's comet (1892 I.) yields the 
still higher value of 40.5 (Fig. 34). Some comets show 
several tails of different kinds, as the famous comet of 
Donati (Fig. 35). Its two almost straight tails would 
belong to the first class, and the more strongly developed 
and curved third tail to the second class. 

Schwarzschild, as already stated, calculated that small 
spherules reflecting all the incident light and of the 

Fig. 35. Donati's comet at its greatest brilliancy in 1858 

specific gravity of water would be repelled by the sun 
with a force that might balance ten times their weight. 
For a spherule absorbing all the light falling upon it 



this figure would be reduced to five times the weight. 
The small particles of comets which, according to spec- 
troscopic observations, probably consist of hydrocarbons 
are not perfectly absorbing, but they permit certain rays 
of the sun to pass. A closer calculation shows that in 
this case forces of about 3.3 times the gravity would 

Larger spherules yield smaller values. Bredichin's sec- 
ond and third classes would thus be well adapted to meet 
the requirements which the radiation pressure demands. 

It is more difficult to explain how such great forces 
of repulsion as those of the first group of Bredichin or of 
the peculiar comets of Swift and of Roerdam can occur. 
When a particle or drop of some hydrocarbon is exposed 
to powerful radiation, it may finally become so intensely 
heated that it will be carbonized. It will yield a spongy 
coal, because gases (chiefly hydrogen) will escape during 
the carbonization, and the particles of coal will resemble 
the little grains of coal-dust which fall from the smoke- 
stacks of our steamboats, and which afterwards float on 
the surface of the water. It is quite conceivable that such 
spherules of coal (consisting probably of so-called mar- 
guerites, felted or pearly structures resembling chains of 
bacilli) may have a specific gravity of 0.1, if we make 
allowance for the gases they include (compare page 
106.) A light-absorbing drop of this density of 0.1 might, 
in the most favorable case, experience a repulsion forty 
times as strong as the gravitation of the sun. In this 
manner we can picture to ourselves the possibility of 
the greatest observed forces of repulsion. 

The spectra of comets confirm in every respect the 
conclusions to which the theory of -the radiation pressure 
leads up. They display a faint, continuous spectrum 

which is probably due to sunlight reflected by the small 
8 103 


particles. Besides this, we observe, as already men- 
tioned, a spectrum of gaseous hydrocarbons and cyano- 
gen. These band spec- 
tra are due to electric 
discharges ; for they are 
observed in comets 
whose distance from the 
sun is so great that 
they cannot appear lu- 
minous owing to their 
own high temperatures. 
In the tail of Swift's 
comet banded spectra 
have been observed in 
portions which were 
about five million kil- 
ometres from the nu- 
cleus. The electric dis- 
charges must chiefly be 
emitted from the outer 
parts of the tails, where, 
according to the laws 
of static electricity, the 
electric forces would be 
strongest. For this rea- 
son the larger tails of 
comets look as if they 
Fig. 36. Imitation of comets' tails, were enveloped in cloaks 

Experiment by Nichols and Hull. of jj ht of & more in _ 

The light oi an arc-lamp is concen- , 

trated by a lens upon the stream of tense luminosity. 

finely powdered particles When a comet Comes 

nearer to the sun, other 

less volatile bodies also begin to evaporate. We then 
find the lines of sodium and, when the comet comes very 



close to the sun, also the lines of iron in its spectrum. 
These lines are evidently produced by substances which 
have been evaporated from the nucleus of the comet. 
Like the meteorites falling upon our earth, the nucleus 
will consist essentially of silicates, and particularly of 
the silicate of sodium, and, further, of iron. 

We can easily imagine how the tails of comets change 
in appearance. When a comet draws near to the sun, 
we observe that matter is ejected from that part of 
the nucleus which is turned towards the sun. The 
case is analogous to the formation of clouds in the ter- 
restrial atmosphere on a hot summer day. The clouds 
are provided with a kind of hood which envelops like a 
thin, semi-spherical veil that side of the nucleus which 
turns to the sun. Sometimes we observe two or more 
hoods corresponding to the different layers of clouds in 
the terrestrial atmosphere. From the farther side of the 
hood matter streams away from the sun. The tails of 
comets are usually more highly developed when they ap- 
proach the sun than when they recede from it. That may 
be, as- has been assumed for a long time, because a large 
part of the hydrocarbons will become exhausted while the 
comet passes the sun. We have also noticed that the 
so-called periodical comets, which return to the sun at 
regular intervals, showed at every reappearance a fainter 
development of the tail. Comets, further, shine at their 
greatest brilliancy in periods of strong solar-spot activity. 
We may, therefore, assume that in those periods the sur- 
roundings of the sun are charged to a relatively high 
degree with the fine dust which can serve as a condensa- 
tion nucleus for the matter of the comets' tails. It is 
also probable that in such periods the ionizing radiation 
of the sun is more pronounced than usual, owing to the 
simultaneous predominance of faculse. 



Nichols and Hull have attempted to imitate tails of 
comets. They heated the spores of the fungus Lycoperdon 
bovista, which are almost spherical and of a diameter 
of about 0.002 mm., up to a red glow, and they thus pro- 
duced little spongy balls of carbon of an average density 
of 0.1. These they mixed with emery-powder and intro- 
duced them into a glass vessel resembling an hour-glass 
(Fig. 36) from which the air had previously been ex- 
hausted as far as possible. They then caused the pow- 
dered mass to fall in a fine stream into the lower part of 
the vessel while exposing it at the same time to the con- 
centrated light of an arc-lamp. The emery particles fell 
perpendicularly to the bottom, while the little balls of 
carbon were driven aside by the radiation pressure of the 

We also meet with the effects of the radiation pressure 
in the immediate neighborhood of the sun. The rectilinear 
extension of the corona streamers to a distance which 
has been known to exceed six times the solar diameter 
(about eight million km.) indicates that repelling forces 
from the sun are acting upon the fine dust. Astronomers 
have also compared the corona of the sun with the tails 
of comets, and Donitsch would class it with Bredichin's 
comets' tails of the second class. It is possible to calcu- 
late the mass of the corona from its radiation of heat and 
light. The heat radiated has been measured by Abbot. 
At a distance of 30,000 km. from the photosphere, the 
corona radiated only as little heat as a body at 55 Cent. 
The reason is that the corona in these parts consists of an 
extremely attenuated mist whose actual temperature can 
be estimated by Stefan's law at 4300 Cent. The corona 
must, therefore, be so attenuated that it would only 
cover a 190,000th part of the sky behind it. We arrive 
at the same result when we calculate the amount of 



light radiated by the corona; this radiation is of the 
order of that of the full moon, being sometimes smaller, 
sometimes greater, up to twice as great. The consider- 
ations we have been offering apply to the most intense 
part of the corona, the so-called inner corona. Accord- 
ing to Turner, its light intensity outward diminishes in 
the inverse ratio of the sixth power of its distance from 
the centre of the sun. At the distance of a solar radius 
(690,000 km.) the light intensity would therefore be only 
1.6 per cent, of the intensity near the surface of the 

Let us assume that the matter of the corona consists 
of particles of just such a size that the radiation pressure 
would balance their weight (other particles would be ex- 
pelled from the inner corona); then we find that the 
weight of the whole corona of the sun would not exceed 
twelve million metric tons. That is not more than the 
weight of four hundred of our large ocean steamships 
(e.g., the Oceanic), and only about as much as the quan- 
tity of coal burned on the earth within one week. 

That the mass of the corona must be extremely rarefied 
has already been concluded, from the fact that comets 
have wandered through the corona without being visibly 
arrested in their motion. In 1843 a comet passed the 
sun's surface at a distance of only one-quarter the sun's 
radius without being disturbed in its progress. Moulton 
calculated that the great comet of 1881, which approached 
the sun within one-half its radius, did not encounter a 
resistance of more than one-fifty-thousandth of its mass, 
and that the nucleus of the comet was at least five million 
times denser than the matter of the corona. Newcomb 
has possibly expressed the degree of attenuation of the 
corona in a somewhat exaggerated way when he said 
that it contains perhaps one grain of dust per cubic kilo- 



metre (a cube whose side has a length of three-fifths of 
a mile). 

However small the quantity of matter in the corona 
may be, and however unimportant a fraction of this mass 
may pass into the coronal rays, it is yet certain that there 
is a constant loss of finely divided matter from the sun. 
The loss, however, is not greater than the supply of mat- 
ter (compare below) -namely, about 300 thousand mill- 
ions of tons in a year so that during one billion years 
not even one-six-thousandth of the solar mass (2X10 27 
tons) will be scattered into space. This number is very 
unreliable, however. We know that many meteorites fall 
upon the earth, partly as compact stones, partly as the 
finest dust of shooting-stars which flash up in the terres- 
trial atmosphere rapidly to be extinguished. These masses 
may be estimated at about 20,000 tons per year. Accord- 
ing to this estimate, the rain of meteorites which falls upon 
the sun may amount to 300 thousand millions of tons in a 
year. All the suns have emitted matter into space for 
infinite ages, and it seems, therefore, a natural inference 
that many suns would no longer be in existence if there 
had not been a supply of matter to make up for this 
loss. The cold suns undergo relatively small losses, but 
receive just as large inflows of matter as the warm suns. 
As, now, our sun belongs to the colder type of stars, it 
is probable that the loss of matter from the sun has for 
this reason been overestimated by being presumed to be 
as great as the accession. The presence of dark celestial 
bodies may compensate for this overestimation. 

Whence do the meteorites come ? If they were not 
constantly being created, their number should have 
diminished in the course of ages; for they are gradually 
being caught up by the larger celestial bodies. It is not 
at all improbable that they arise from the accrescence 



of small particles which the radiation pressure has been 
driving out of the sun. The chondri, which are so char- 
acteristic of meteorites, display a structure as if they 
had grown together out of a multitude of extremely 
fine grains (Fig. 37). Nordenskio'ld says: "Most meteoric 
iron consists of an extremely delicate texture of various 

Fig:. 37. Granular chondrum from the meteorite of Sexes. 
Enlargement 1 : 70. After S. Tschermak 

alloys of metals. This mass of meteoric iron is often so 
porous that it oxidizes on exposure to the air like spongy 
iron. The Pallas iron, when cut through with a saw, shows 
this property, which is so distressing for the collector. The 



iron of Cranbourne, of Toluca, and others in fact, almost 
all the meteorites with a few exceptions display the same 
texture. It all indicates that these cosmical masses of 
iron were built up in the universe by particle being piled 
upon particle, of iron, nickel, phosphorus, etc., analogous 
to the manner in which one atom of a metal coalesces 
with another atom when the metal is galvanically de- 
posited from a solution. Most of the stony meteorites pre- 
sent a similar appearance. Apart from the crust of slag 
on the surface, the stone is often so porous and so loose 
that it might be used as a filtering material, and it may 
easily be crumbled between the fingers." When the 
electrically charged grains of dust coalesce, their small 
electrical potential (of about 0.02 volt) may increase 
considerably. Under the influence of ultra-violet light 
these masses of meteorites are discharged when they ap- 
proach the sun, as Lenard has shown. Their negative 
charge then escapes in the shape of so-called electrons. 
Since, now, the sun loses through the rays of the 
corona large multitudes of particles, and these particles 
probably carry, according to Wilson, negative electricity 
with them, the positive charge must remain behind in 
the stratum from which the coronal rays were emitted, 
and also on the sun itself. If this charge were sufficiently 
powerful, it would prevent the negatively charged par- 
ticles in the corona from escaping from the sun, and all 
the phenomena which we have ascribed to the radiation 
pressure would cease. By the aid of the tenets of the 
modern theory of electrons, I have calculated the maxi- 
mum charge that the sun could bear, if it is not to stop 
these phenomena. The charge would amount to two 
hundred thousand millions of coulombs not by any 
means too large a quantity of electricity, as it would only 

be sufficient to decompose twenty-four tons of water. 



By means of this positive charge the sun. exerts a 
vast attractive power upon all negatively charged par- 
ticles which come near it. We have already remarked 
that the grains of sun-dust which have united to form 
meteorites lose under the influence of ultra-violet light 
their charge in the shape of negative electrons, ex- 
tremely minute particles, of which perhaps one thousand 
weigh as much as one atom of hydrogen (1 gramme of 
hydrogen contains about 10 24 atoms, corresponding, to 
10 27 electrons). These electrons wander about in space. 
When they approach a positively charged celestial body 
they are attracted by it with great force. If the electrons 
were moving with a velocity of 300 km. per second, as in 
Lenard's experiment, and if the sun were charged to 
one-tenth the maximum amount just calculated, it would 
be able to draw up all the electrons whose rectilinear 
path (so far as not curved by the sun's attraction) would 
lie at a distance from the sun 125 times as great as the 
distance between the sun and its most remote planet, 
Neptune, and 3800 times as great as the distance between 
the sun and the earth, which, after all, would only be one- 
sixtieth of the distance from our nearest fixed-star neigh- 
bor. The sun drains, so to speak, its surroundings of 
negative electricity, and this draining effect carries to 
the sun, as could easily be proved, a quantity of electric- 
ity which is directly dependent upon the positive charge 
of the sun. Thus, so far as electricity is concerned, 
ample provision has been made for maintaining equilib- 
rium between the income and expenditure of the sun. 

When an electrical particle enters into a magnetic field 
it describes a spiral about the so-called magnetic lines of 
force; when at a greater distance, the particles appear 

to move in the direction of the lines themselves. The 



rays of the corona emanating from the solar poles show 
a distinct curvature like that of the lines of force about 
a magnet, and for this reason the sun has been regarded 
as a big magnet whose magnetic poles nearly coincide 
with the geographical poles. The coronal rays nearer 
the equator likewise show this curvature (compare Fig. 
30). The repelling force of the radiation pressure there 
is, however, at right angles to the lines of force and much 
stronger than the magnetic force, so that the rays of the 
corona are compelled to form two big streams flowing in 
the equatorial direction. This is especially noticeable at 
times of sun-spot minima. During the times of sun-spot 
maxima the strength of the radiation pressure of the 
initial velocity of the grains of dust seem to predom- 
inate so markedly that the magnetic force is relatively 

The astronomers tell us that the sun is only a star of 
small light intensity compared to the prominent stars 
which excite our admiration. The sun further belongs 
to a group of relatively cold stars. We may easily im- 
agine, therefore, that the radiation pressure in the vicin- 
ity of these larger stars will be able to move much larger 
masses of matter than in our solar system. If the dif- 
ferent stars had at any time consisted of different chemi- 
cal elements, this difference would have been equalized 
in the course of ages. The meteorites may be regarded 
as samples of matter collected and despatched from all 
possible divisions of space. Now, what bodies do we 
find in them? 

In. the comets (compare page 104) iron, sodium, car- 
bon, hydrogen, and nitrogen (as cyanogen) play the most 
important part. We know, especially from the researches 
of Schiaparelli that meteorites often represent fragments 

of comets, and must therefore be related to them. Thus 



Biela's comet, which had a period oi 6.6 years, has dis- 
appeared since 1852 it had divided into two parts in 
1844 -- 1845. The comet was rediscovered in a belt of 
meteorites of the same period which approaches the orbit 
of the earth each year on November 27. Similar rela- 
tions have been observed with regard to several other 
swarms of meteorites. We know also that the just-men- 
tioned elements which spectrum analysis has proved to 
exist in comets are the main constituents of the meteor- 
ites, which, in addition, contain the metals calcium, mag- 
nesium, aluminium, nickel, cobalt, and chromium, as 
well as the metalloids oxygen, silicon, sulphur, phos- 
phorus, chlorine, arsenic, argon, and helium. Their 
composition strongly recalls the volcanic products of 
so-called basic nature that is to say, those which con- 
tain relatively large proportions of metallic oxides, and 
which have been thought for good reasons to hail from 
the deeper strata of the interior of the earth. Lockyer 
heated meteoric stones in the electric arc to incandescence 
and found their spectra to be very similar to the solar 

We therefore draw the conclusion that these messen- 
gers from other solar systems which bring us samples 
of their chemical elements are closely related to our sun 
and to the interior of our earth. That other stare and 
comets are essentially composed of the same elements as 
our sun and earth, spectrum analysis had already in- 
timated to us. But various metalloids, like chlorine, 
bromine, sulphur, phosphorus, and arsenic, which are of 
importance for the composition of the earth, have so 
far not been traced in the spectra of the celestial bodies, 
nor in that of the sun. W^e find them in meteorites, 
however, and there is not the slightest doubt that we 
must likewise count them among the essential constitu- 



ents of the sun and other celestial bodies. It is with diffi- 
culty, however, that the metalloids can be made to ex- 
hibit their spectra, and this is manifestly the reason why 
spectrum analysis has not yet succeeded in establishing 
their presence in the heavens. As regards the recently 
discovered so-called noble gases helium, argon, neon, kryp- 
ton, and xenon, their presence in the chromosphere has 
been discovered on spectrograms taken during eclipses of 
the sun (Stassano). According to Mitchell, however, 
these statements would appear to be somewhat uncer- 
tain as to krypton and xenon. 

The small particles of dust which the radiation press- 
ure drives out into space to all possible distances from 
the sun and the stars may hit against one another and 
may accumulate to larger or smaller aggregates in the 
shape of cosmical dust or meteorites. These aggregates 
will partly fall upon other stars, planets, comets, or 
moons, and partly and this in very great multitudes 
they will float about in space. There they may, together 
with the larger dark celestial bodies, form a kind of 
haze, which partly hides from us the light of distant 
celestial bodies. Hence we do not see the whole sky 
covered with luminous stars, which would be the case 
if, as we may surmise, the stars were uniformly distrib- 
uted all through the infinite space of the universe, and 
if there were no obstacle to their emission of light. If 
there were no other celestial bodies of very low tempera- 
ture and very large dimensions which absorbed the heat 
of the bright suns, the dark celestial bodies, the meteor- 
ites, and the dark cosmical dust would soon be so strongly 
heated by solar radiation that they would themselves 
turn incandescent, and the whole dome of the sky would 
appear to us like one glowing vault whose hot radiation 
down to the earth would soon burn every living thing. 



These other cold celestial bodies which absorb the 
solar rays without themselves becoming hot are known as 
nebulae. More recent researches make us believe that 
these peculiar celestial bodies occur nearly everywhere in 
the sky. The wonderful mechanism which enables them 
to absorb heat without raising their own temperature will 
be explained later (in Chapter VII.). As these cold nebu- 
lae occupy vast portions of space, most of the cosmical 
dust must finally, in its wanderings through infinite space, 
stray into them. This dust will there meet masses of 
gases which stop the penetration of the small corpuscles. 
As the dust is electrically charged (particularly with nega- 
tive electricity), these charges will also be accumulated 
in the outer layers of the nebulae. This will proceed until 
the electrical tension becomes so strong that discharges 
are started by the ejection of electrons. The surround- 
ing gases will therefore be rendered luminescent, although 
their temperature may not much (perhaps by 50) exceed 
absolute zero, 273 Cent., and in this way we are en- 
abled to observe these nebulae. Most of the particles 
will be stopped before they have had time to penetrate 
very deeply into a nebula, and it will therefore principally 
be the outer portions of the nebulae which send their 
light to us. That would conform to Herschel's descrip- 
tion of planetary nebulae, which display no greater lu- 
minosity in their centres, but which shine as if they 
formed hollow spherical shells of nebulous matter. It 
is very easy to demonstrate that only substances, such as 
helium and hydrogen, which are most difficult to condense, 
can at this low temperature exist in gaseous form to any 
noticeable degree. The nebulae, therefore* shine almost 
exclusively in the light of these gases. There occurs in 
the nebulae, in addition to these gases, a mysterious sub- 
stance, nebulium, whose peculiar spectrum has not been 



found on the earth nor in the light of stars. For- 
merly the character of the nebular spectrum was ex- 
plained by the assumption either that no other bodies 
occurred in nebulae than the substances mentioned, or 
that all the other elements in them were decomposed 
into hydrogen helium was not known then. The sim- 
ple explanation is that only the gases of the outer layer 
of the nebulae are luminous. How their interiors are con- 
stituted, we do not know. 

It has been objected to the view just expressed that 
the whole sky should glow in a nebulous light, and that 
even the outer atmosphere of the earth should display 
such a glow. But hydrogen and helium occur only very 
sparely in the terrestrial atmosphere. We find, how- 
ever, another light, the so-called auroral line, which may 
possibly be due to krypton in our atmosphere. Which- 
ever way we turn the spectroscope on a very clear night, 
especially in the tropics, we observe this peculiar green 
line. It was formerly considered to be characteristic of 
the Zodiacal Light, but on a closer examination it has 
been traced all over the sky, even where the Zodiacal 
Light could not be observed. One of the objections to 
our view is therefore unjustified. 

As regards the other objection, we have to remark 
that any light emission must exceed a certain minimum 
intensity to become visible. There may be nebulae, and 
they probably constitute the majority, which we cannot 
observe because the number of electrically charged par- 
ticles rushing into them is far too insignificant. A con- 
firmation of this view was furnished by the flashing-up 
of the new star in Perseus on February 21 and 22, 1901. 
This star ejected two different kinds of particles, of which 
the one kind travelled with nearly double the velocity of 
the other. The accumulations of dust formed two spher- 



ical shells around the new star, corresponding in every 
respect to the two kinds of comets' tails of Bredichin's 
first and second classes, which we have sometimes ob- 
served together in the same comet (Fig. 35). When these 
dust particles, on their road, hit against nebular masses, 
the latter became luminescent, and we thereby obtained 
knowledge of the presence of large stellar nebulae of 
whose existence we previously had not the faintest 
suspicion. Conditions, no doubt, are similar in other 
parts of the heavens where we have not so far discovered 
any nebulae we believe, because of the small number of 
these charged particles straying about in those parts. 
On the same grounds we may explain the variability of 
certain nebulae which formerly appeared quite enigmatical. 



WE have so far dwelt on the effects which the particles 
expelled from the sun and the stars exert on distant 
celestial bodies. It may be asked whether this dust 
does not act upon our own earth. We have already 
recognized the peculiar luminescence which on clear 
nights is diffused over the sky as a consequence of elec- 
trical discharges of this straying dust. This leads to the 
question whether the magnificent polar lights, which 
according to modern views are also caused by electric 
discharges in the higher strata of the atmosphere, are not 
produced by dust which the sun sends to us. It will, 
indeed, be seen that we can in this way explain quite a 
number of the peculiarities of these mysterious phenomena 
which have always excited man's imagination. 

We know that meteorites and shooting-stars are ren- 
dered incandescent by the resistance which they encounter 
in the air at an average height of 120 km. (75 miles), some- 
times of 150 and 200 km. In isolated cases meteorites 
are supposed to have become visible even at still greater 
altitudes. It would result that there must be appre- 
ciable quantities of air still at relatively high elevation, 
and that the atmosphere cannot be imperceptible at an 
altitude of less than 100 km., as was formerly assumed. 
Bodies smaller than the meteorites as well as the solar dust 



we have spoken of which, owing to their minuteness and 
to the strong cooling by heat radiation and conduction 
that they undergo in passing through the atmosphere, 
could never attain incandescence would be stopped at 
greater heights. We will assume that they are arrested 
at a mean height of about 400 km. (250 miles). 

The masses of dust which are expelled by the sun are 
partly uncharged, partly charged with positive or nega- 
tive electricity. Only the latter can be connected with 
the polar lights; the former would remain dark and slow- 
ly sink through our atmosphere to the surface of the earth. 
They form the so-called cosmical dust, of whose great 
importance Nordenskiold was so firmly convinced. He 
estimated that the yearly increase in the weight -of the 
earth by the addition of the meteorites was at least ten 
million tons, or five hundred times more than we stated 
above (page 108). Like Lockyer and, in more recent 
days, Chamberlin, he believed that the planets were 
largely built up of meteorites. 

The dust reaching the earth from the sun would not, 
were it not electrically charged, amount to more than 
200 tons in a year. Although this figure may be far too 
low, yet the supply of matter by these means is certainly 
very small in comparison with the 20,000 tons which the 
earth receives in the shape of meteorites and shooting- 
stars. But owing to its extremely minute distribution, 
the effect of this dust is very important, and it may con- 
stitute a much greater portion of the finely distributed 
cosmical dust in the highest strata of the atmosphere than 
the dust introduced by falling meteorites and shooting- 

That these particles exert a noticeable influence upon 
terrestrial conditions, in spite of their relatively insig- 
nificant mass, is due to two causes. They are extremely 

9 119 


minute and therefore remain suspended in our atmos- 
phere for long periods (for more than a year in the case 
of the Krakatoa dust), and they are electrically charged. 

In order to understand their action upon the earth, 
we will examine how the terrestrial conditions depend 
upon the position of the earth with regard to the various 
active portions of the sun, and upon the change of the 
sun itself in regard to its emission of dust particles. For 
this examination we have to avail ourselves of extensive 
statistical data; for only a long series of observations can 
give us a clear conception of the action of solar dust. 

These particles withdraw from the sun gases which 
they were able to condense on their surface, and which 
had originally been in the chromosphere and in the 
corona of the sun. The most important among these 
gases is hydrogen ; next to it come helium and the other 
noble gases which Ramsay has discovered in the atmos- 
phere, in which they occur in very small quantities. As 
regards hydrogen, Liveing and (after him) Mitchell have 
maintained that it is not produced in the terrestrial at- 
mosphere. Occasionally it is certainly found in volcanic 
gases. Thus hydrogen escapes, for instance, from the 
crater of Kilauea, on Hawaii, but it is burned at once in 
the atmosphere. If hydrogen were present in the at- 
mosphere, it would gradually combine with the oxygen 
to water vapor; and we have to assume, therefore, that 
the hydrogen must be introduced into our atmosphere 
from another source namely, from the sun. Mitchell 
finds in this view a strong support for the opinion that 
solar dust is always trickling down through our atmos- 

The quantity of solar dust which reaches our atmos- 
phere will naturally vary in proportion with the erup- 
tive activity of the sun. The quantity of dust in the 



higher strata influences the color of the light of the sun. 
After the eruption of the volcano Rakata on Krakatoa, 
in 1883, and again, though to a lesser degree, after the 
eruption of Mont Pelee on Martinique, red sunsets and 
sunrises were observed all over the globe. At the same 
time, another phenomenon was noticed which could be 
estimated quantitatively. The light of the sky is polar- 
ized with the exception of the light coming from a few 
particular spots. Of these spots, one called Arago's Point 
is situated a little above the antipode of the sun, and 
another, Babinet's Point, is situated above the sun. If 
we determine the elevation of these points above the hori- 
zon at sunset, we find in accordance with the theoretical 
deduction that this elevation is greater when the higher 
strata of the atmosphere are charged with dust (as after 
the eruption of Rakata) than under normal conditions. 
Busch, a German scientist, analyzed the mean elevation 
of these points (stated in degrees of arc) at sunset, and 
found the following peculiar numbers: 












Arago's Point . . . 












Babinet's Point . . 












Sun-spot Number 












There is a distinct parallelism in these series of figures. 
Almost simultaneously with the sun-spot maximum the 
height of the two so-called neutral points above the 
horizon attains its maximum at sunset, and the same 
applies to the minimum. That the phenomena in the 
atmosphere take place a little later than the phenomena 
on the sun which caused them is perhaps only natural. 
When the air is rich in dust, or when it is strongly 
ionized by kathode rays, conditions are favorable for 
the formation of clouds. This can be observed, for in- 



stance, with auroral lights. They regularly give rise to 
a characteristic cloud formation, so much so that Adam 
Paulsen was able to recognize polar lights by the aid of 
these clouds in full daylight. Klein has compiled a table 
on the connection between the frequency of the higher 
clouds, the so-called cirrus clouds, at Cologne, and the 
number of sun-spots during the period 1850-1900. He 
demonstrates that during this half-century, which com- 
prises more than four sun-spot periods, the sun-spot 
maxima fell in the years in which the greatest number of 
cirrus clouds had been observed. The minima of the 
two phenomena are likewise in agreement. 

A similarly intensified formation of clouds seems also 
to occur on Jupiter when sun-spots are frequent. Vogel 
states that Jupiter at such times shines with a whiter 
light, while at sun-spot minima it appears of a deeper 
red. The deeper we are able to peep into the atmos- 
phere of Jupiter, the more reddish it appears. During 
periods of strong solar activity the higher portions of 
Jupiter's atmosphere therefore appear to be crowded 
with clouds. 

The discharge of the charged solar dust in our atmos- 
phere calls forth the polar lights. 

The polar lights occur, as the name indicates, most fre- 
quently in the districts about the poles of the earth. They 
are, however, not actually more frequent the nearer we 
come to the poles; but they attain a maximum of fre- 
quency in circles which enclose the magnetic and the 
geographical poles. The northern maximum belt passes, 
via Cape Tscheljuskin, north of Novaja Semi j a, along the 
northwestern coast of Norway, a few degrees to the south 
of Iceland and Greenland, right across Hudson Bay 'and 
over the northwestern extension of Alaska. When we 
go to the south of this belt, the auroras, or boreal lights, 



diminish markedly. They are four times less frequent 
in Edinburgh, and fifteen times less frequent in London 
or New York, than in the Orkney Islands or Lab- 

Paulsen divides the auroras into two classes, which 
behave quite differently in several respects. The great 
difficulties which the solution of the problems of polar 
lights has so far offered seem to a large extent to be 
due to the fact that all polar lights were treated as 
being of the same kind. 

The polar lights of the first class do not display any 
streamers. They cover a large portion of the sky in a 
horizontal direction. They are very quiet, and their 
light is strikingly constant. As a rule, they drift slowly 
towards the zenith, and they do not give rise to any 
magnetic disturbances. 

These polar lights generally have the shape of an 
arch whose apex is situated in the direction of the mag- 
netic meridian (Fig. 38). Sometimes several arches are 
grouped one above another. 

Nordenskiold observed these arches quite regularly 
during the polar night when he was wintering near Pit- 
lekaj, in the neighborhood of Bering Sound. Adam 
Paulsen has often seen them on Iceland and Greenland, 
which are situated within the maximum belt spoken of, 
where northern lights are very common. Occasionally, 
auroras are also seen farther from the poles, as cir- 
cular arches of a milky white, which may be quite 
high in the heavens. 

Sometimes we perceive in the arctic regions that large 
areas of the heavens are covered by a diffused light which 
might best be compared to a luminous, transparent cloud ; 
the darker portions in it probably appear dark by con- 
trast. This phenomenon was frequently observed dur- 



Fig. 38. Arch-shaped aurorae borealis, observed by 
Nordenskiold during the wintering of the Vega in 
Bering Strait, 1879 

ing the Swedish expedition of 1882-1883, near Cape 

Masses of light at so low a level that the rocks behind 
them are obscured have frequently been observed to float 
in the air, especially in the arctic districts. Thus Lem- 
strom saw an aurora on the island of Spitzbergen in front 
of a wall of rock only 300 m. (1000 ft.) in height. In 
northern Finland he observed the auroral line in the light 
of the air in front of a black cloth only a few metres dis- 
tant. Adam Paulsen counts these phenomena also as 
polar lights of the first class, and he regards them as 



phosphorescent clouds which have been carried down by 
convection currents to an unusually low level of our at- 

Polar lights of the second class are distinguished by 
the characteristic auroral rays or streamers. Sometimes 
these streamers are quite separated from one another 
(see Fig. 39); as a rule they melt into one another, es- 
pecially below, so as to form draperies which are so easily 
moved and unsteady that they appear to flutter in the 

Fig. 39. Aurora borealis, with radial streamers 

wind (Fig. 41.) The streamers run very approximately 
in the direction of the inclination (magnetic dip) needle, 
and when -they are fully developed around the celestial 
dome their point of convergence is distinctly discernible 
in the so-called corona (Fig. 40). When the light is at 
its greatest intensity the aurora is traversed by numer- 
ous waves of light. 

The draperies are very thin. Paulsen watched them 
sometimes drifting over his head in Greenland. The dra- 
peries then appeared foreshortened, in the shape of striae or 



ribbons of light in convolutions. These polar lights in- 
fluence the magnetic needle. When they pass the zenith 
their influence changes sign, so that the deviation of the 
magnetic needle changes from east to west when the rib- 

Fig. 40. Aurora with corona, observed by 
Gyllenskiold on Spitzbergen, 1883 

bon is moving from north to south. Paulsen therefore 
concluded that negative electricity (kathode rays) was 
moving downward in these rays. These polar lights cor- 
respond to violent displacements of negative electric- 
ity, while polar lights of the first class appear to consist 
of a phosphorescent matter which is not in strong agita- 
tion. The streamers may penetrate down into rather 
low atmospheric strata, at least in districts which are 
near the maximum belt of the northern lights. Thus 
Parry observed at Port Bowen an auroral streamer in 
front of a cliff only 214 m. (700 ft.) in height. 

Polar lights of the first order may pass into those 
of the second order, and vice versa. We frequently see 
rays suddenly flash out from the arch of the aurora, most- 
ly downward, but, when the display is very intense, also 
upward. On the other hand, the violent agitation of a 



"drapery light" may cease, and may give way to a dif- 
fused, steady glow in the sky. The polar light of the first 
class is chiefly observed in the arctic regions. To it cor- 
responds, in districts farther removed from the pole, the 
diffused light which appears to be spread uniformly over 
the heavens and which gives the auroral line. 

The usually observed polar lights (speaking not only 
of those seen on arctic expeditions) belong to the second 
class, which comprises also all those included in the sub- 
joined statistics, with the exception of the auroral dis- 
plays reported from Iceland and Greenland. While the 

Fig. 41. Polar-light draperies, observed in Finnmarken, northern 


streamer lights distinctly conform to the 11.1 years' 
period, and become more frequent at times of sun-spot 
maxima, this is not the case, according to Tromholt, 
with the auroras of Iceland and Greenland. Their fre- 



quency, on the contrary, seems to be rather independent 
of the sun-spot frequency. Not rarely auroral maxima 
corresponding to sun-spot maxima are subdivided into 
two by a secondary minimum. This phenomenon is 
most evident in the polar regions, but it can also be 
traced in the statistics from Scandinavia and from other 

Better to understand the nature of auroras, we will 
consider the sun's corona during the time of a minimum 
year, taking as an example the year 1900 (compare Fig. 
30). The rays of the corona in the neighborhood of the 
poles of the sun are laterally deflected by the action of 
the magnetic lines of force of the sun. The small, nega- 
tively charged particles have evidently only a low veloc- 
ity, so that they move quite close to the lines of force in 
the neighborhood of the solar poles and are concentrated 
near the equator. There the lines of force are less crowded 
that is to say, the magnetic forces are weaker and 
the solar dust can therefore be ejected by the radiation 
pressure and will accumulate to a large disk expanding 
in the equatorial plane. To us this disk appears like two 
large streams of rays which project in the direction of the 
solar equator. Part of this solar dust will come near the 
earth and be deflected by the magnetic lines of force 
of the earth; it will hence be divided into two streams 
which are directed towards the two terrestrial magnetic 
poles. These poles are situated below the earth's crust, 
and therefore not all the rays will be concentrated towards 
the apparent position of the magnetic poles upon the 
surface of the earth. It is to be expected that the nega- 
tively charged particles coming from the sun will chiefly 
drift towards that district which is situated somewhat 
to the south of the magnetic north pole, when it is noon 
at this pole. When it is midnight at the magnetic 



pole, most of the negatively charged particles will be 
caught by the lines of force before they pass the geo- 
graphical north pole, and the maximum belt of the auroras 
will for this reason surround the magnetic and the geo- 
graphical poles, as has already been pointed out (com- 
pare page 122). The negatively charged solar dust will 
thus be concentrated in two rings above the maximum 
belts of the polar lights. Where the dust collides with 
molecules of the air, it will produce a phosphorescent 
glow, as if these molecules were hit by the electrically 
charged particles of radium. This phosphorescent glow 
rises in the shape of a luminous arch to a height of 
about 400 km. (250 miles) according to Paulsen and 
the apex of this arch will in every part seem to lie in 
the direction where the maximum belt is nearest to the 
station of the observer. That will fairly coincide with 
the direction of the magnetic needle. 

The solar corona of a sun-spot maximum year is of a 
very different appearance (Fig. 31). The streamers radi- 
ate straight from the sun in almost all directions; and if 
there be some privileged directions, it will be those above 
the sun-spot belts. The velocity of the solar dust is evi- 
dently so great that the streamers are no longer visibly 
deflected by the magnetic lines of force of the sun. Nor 
is this charged dust influenced to any noticeable degree 
by the magnetic lines of force of the earth. It will in 
the main fall straight down in that part of the atmos- 
phere in which the radiation is most intense. As these 
"hard" rays of the sun 1 seem to issue from the faculse 
of the sun which are most frequent in maximum sun-spot 

1 The designations "hard" and "soft" streams of solar dust cor- 
respond to the terms used with regard to kathode rays. The soft 
rays have a smaller velocity, and are therefore more strongly deflected 
by external forces, as, for instance, magnetic forces. 



years, some polar lights will also be seen in districts 
which are far removed from the maximum belt of the 
auroras, especially when the number of sun-spots is 
large. The opposite relation holds for the " soft" streams 
of solar dust which fall near the maximum belt of the 
polar lights. These streams occur most frequently with 
low sun-spot frequency, as we know from observations of 
the solar corona. Possibly they are carried along by the 
stream of harder rays in maximum years. The polar 
lights corresponding to these rays therefore attain their 
maximum with few sun-spots. Hard and soft dust 
streams occur, of course, simultaneously; but the former 
predominate in maximum sun-spot years, the latter in 
minimum years. 

That the periodicity of the polar lights in regions with- 
out, the maximum belt follows very closely the periodicity 
of the sun-spots was shown by Fritz as early as 1863. 
The length of the period varies between 7 and 16 years, 
the average being 11.1 years. The years of maxima and 
minima for sun-spots and for northern auroras are the 
following : 


Sun-spots 1728 '39 '50 '62 '70 '78 '88 1804 '16 '30 

1837 '48 '60 '71 '83 '93 1905 

Northern lights... 1730 '41 '49 '61 '73 '78 '88 1805 '19 '30 

1840 '50 '62 '71 '82 '93 1905 


Sun-spots 1734 '45 '55 '67 '76 '85 '98 1811 '23 '34 

1844 '56 '67 '78 '89 1900 

Northern lights... 1735 '44 '55 '66 '75 '83 '99 1811 '22 '34 

1844 '56 '66 '78 '89 1900 

There are, in addition, as De Mairan proved in his 
classical memoir of the year 1746, longer periods common 



to both the number of sun-spots and the number of 
auroras. According to Hansky, the length of this period 
is 72 years; according to Schuster, 33 years. Very pro- 
nounced maxima occurred at the beginning and the end 
of the eighteenth century, the last in the year 1788; after- 
wards auroras became very rare in the years 1800-1830, 
just as in the middle of the eighteenth century. In 
1850, and particularly in 1871, there were strong maxima; 
they have been absent since then. 

The estimates of the heights of the polar lights vary 
very considerably. The height seems to be the greater, 
on the whole, the nearer the point of observation is to the 
equator, which would well agree with the slight deflec- 
tion of the kathode rays towards the surface of the 
earth in regions which are farther removed from the pole. 
Gyllenskiold found on Spitzbergen a mean height of 55 
km.; Bravais, in northern Norway, 100 to 200 km; De 
Mairan, in central Europe, 900 km.; Galle, again, 300 
km. In Greenland, Paulsen observed northern lights at 
very low levels. In Iceland he fixed the apex of the north- 
ern arch which may be considered as a point where the 
charged particles from the sun are discharged into the 
air at about 400 km. Not much reliance can be placed 
upon the earlier determinations; but the heights given 
conform approximately to the order of magnitude which 
we may deduce from the height at which the solar dust 
will be stopped by the terrestrial atmosphere; 

The polar lights possess, further, a pronounced yearly 
periodicity which is easily explicable by the aid of the 
solar dust theory. We have seen that sun-spots are 
rarely observed near the solar equator, and the same ap- 
plies to solar faculrc. They rapidly increase in frequency 
with higher latitudes of the sun, and their maximum 
occurs at latitudes of about fifteen degrees. The equa- 



torial plane of the sun is inclined by about seven degrees 
towards the plane of the earth's orbit. The earth is in 
the equatorial plane of the sun on December 6th and 
June 4th, and most distant from it three months later. 
We may, therefore, expect that the smallest number of 
solar-dust particles will fall on the earth when the earth 
is in the equator of the sun that is, in December and 
June and the greatest number in March and Septem- 
ber. These relations are somewhat disturbed by the 
twilight, which interferes with the observation of auroras 
in the bright summer nights of the arctic region, while 
the dark nights of the winter favor the observation of 
these phenomena. The distribution of the polar lights 
over the different seasons of the year will become clear 
from the subjoined table compiled by Ekholm and my- 

January . . 




Iceland and 




at i rone 


February . 
































August. . . 














. . . 1077 





December . 







number. 727 





In zones where the difference between the lengths of 
day and night of the different seasons is not very great, 
as in the United States, and in districts in which the 
southern light is observed (about latitude 40 S.), the 
chief minimum falls in winter: on the northern hemi- 
sphere, in December ; on the southern hemisphere, in June 



or July. A less pronounced minimum occurring in the 
summer. Twice in the course of the year the earth 
passes through the plane of the solar equator. During 
these periods a minimum of solar dust trickles down 
upon the earth, and that period is characterized by a 
larger number of polar lights which is distinguished by a 
higher elevation of the sun above the horizon. We may 
expect this; for most solar dust will fall upon that por- 
tion of the earth over which the sun is highest at noon. 
The two maxima of March or April and of September or 
October, when the earth is at its greatest distance from 
the plane of the solar equator, are strongly marked in 
all the series, except in those for the polar districts Ice- 
land and Greenland. There the auroral frequency is 
solely dependent upon the intensity of the twilight, so 
that we find a single maximum in December and the cor- 
responding minimum in June. More recent statistics 
(1891-1903) indicate, however, a minimum in December. 
For the same reason the summer minimum in countries 
of high latitudes, like Sweden and Norway, is very much 

Similar reasons render it difficult for most localities 
to indicate the daily periodicity of the polar lights. 
Most of the solar dust falls about noon, and most polar 
lights should therefore be counted a few hours after 
noon, just as the highest temperature of the day is 
reached a little after noon. On account of the intense 
sunlight, however, this maximum can only be estab- 
lished in the wintry night of the polar regions, and even 
there only when a correction has been made for the dis- 
turbing effect of the twilight. In this way Gyllenskiold 
found a northern-light maximum at 2.40 P.M. for Cape 
Thordsen, on Spitzbergen, the corresponding minimum 

being at 7.40 A.M. In other localities we can only as- 



certain that the polar lights are more intense and more 
frequent before than after midnight. In central Europe 
the maximum occurs at about 9 P.M.; in Sweden and 
Norway (in latitude 60 N.), half an hour or an hour 

A few other periods, approximately of the length of 
a month, have .been suggested with regard to polar 
lights. A period lasting 25.93 days predominates in the 
southern lights, where the maximum exceeds the aver- 
age by 44 per cent. For the northern lights in Norway 
the corresponding excess percentage is 23; for Sweden, 
only II. 1 

The same period of nearly twenty-six days had already 
been pointed out for a long series of other especially mag- 
netic phenomena which, as we shall see, are very closely 
connected with auroras, and it had also been found in the 
frequency of thunder-storms and in the variations of the 
barometer. This periodicity has often been thought to be 
connected with the axial rotation of the sun. The Aus- 
trian scientist Hornstein has even gone so far as to pro- 
pose that the length of this period should be carefully de- 
termined, " because it would give a more accurate value 
for the rotation of the sun than the direct determinations." 
We know now that the length of the solar revolution is dif- 
ferent for different solar altitudes, a circumstance with 
which observations of sun-spot movements at different 
latitudes had already made Carrington and Sporer famil- 
liar, but which was not safely established before Duner's 
spectroscopical determination of the movement of the solar 

1 The reason is that in the southern district only very few, and 
chiefly the most intense, auroras are recorded. If we observe very 
assiduously in a large country, and conduct the observations at differ- 
ent spots, we shall find polar light almost every night. This consid- 
eration partly wipes out the just-mentioned differences. 



photosphere. Duner found the following sidereal revo- 
lutions for different latitudes of the sun to which the sub- 
joined sy nodical revolution would correspond. (By side- 
real revolution of a point on the sun we understand the 
time which elapses between the two moments when a 
certain star passes, on two consecutive occasions, through 
the meridian plane of the point that is to say, through 
a plane laid through the poles of the sun and the point 
in question. The synodical revolution is determined by 
the passage of the earth through this meridian. On ac- 
count of the proper motion of the earth the synodical 
period is longer than the sidereal period.) 

Latitude on the sun (degrees) ... 15 30 45 60 75 

Sidereal revolution (days) 25.4 26.4 27.6 30.0 33.9 38.5 

Synodical revolution (days) 27.3 28.5 29.9 32.7 37.4 43.0 

That the periods of rotation of the solar photosphere, 
and, in a similar way, the periods of the spots, the faculse, 
and the prominences, should become so considerably 
longer with increasing latitudes is one of the most mys- 
terious problems of the physics of the sun. Something 
similar applies to the clouds of Jupiter, but the differ- 
ence in that case is much smaller only about one per 
cent. The clouds of our atmosphere behave quite dif- 
ferently, a fact which is explained by our atmospheric 
circulation. 1 

In our case, of course, the position of the sun with re- 
gard to the earth that is to say, the synodical period can 

1 The very highest strata of our atmosphere (at levels of from 
20 to 80 km., 15 to 50 miles) may perhaps form an exception. The 
luminous clouds which were observed in the years 1883-1892 at 
Berlin (after the eruption of Krakatoa), and which were floating at 
a very high level, showed a drift with regard to the surface of the 
earth opposite to the drift of the cirrus clouds, which are directed 

' 135 


alone be of importance. We recognize that the period 
of 25.93 days does not at all agree with any period of 
the solar photosphere. The solar equatorial zone differs 
least, and it would be appropriate to reckon with this 
period, since the earth never moves very far from the 
plane of the solar equator, and returns to that plane, at 
any rate, twice in the course of a year. 

But there is another peculiarity. The higher a point 
is situated in the atmosphere of the sun, the shorter is 
its period. Thus the synodical period of the faculse near 
the equator is on an average 26.06, the period of the spots 
26.82, of the photosphere 27.3 days. Faculse situated at 
higher levels revolve still more rapidly, and we are thus 
driven to the conclusion that the period to which we 
have alluded agrees with the period of the faculse situated 
at higher levels in the equatorial zone of the sun, and is 
probably dependent upon them. That would conform 
to our ideas concerning the physics of the sun. For the 
faculse are produced in the ascending currents of gas and 
at rather lower levels than the spherules which are ex- 
pelled by the radiation pressure. This radiation pressure 
is strongest just in the neighborhood of the faculse. 

For the same reason the repulsion of the solar dust 
becomes particularly powerful when the faculse are 
strongly developed that is to say, just in the time of 
pronounced eruptive activity of the sun which is char- 
acterized by many sun-spots. 

We must thus imagine that the radiation of the sun 
will be stronger in times of strong eruptive activity than 
during the days of low sun-spot frequencies. Direct ob- 
servations of the intensity of the solar radiation which 
have been made by Saveljeff in Kieff confirm this assump- 
tion. It must be pointed out, however, that another 
phenomenon investigated by Koppen seems to contradict 



this conclusion. Koppen ascertained that in our tropics 
the temperature was by 0.32 Cent, (nearly 0.6 F.) 
lower during sun - spot maxima than the average, and 
that five years later, a year before the sun-spot mini- 
mum, it reached its maximum value of 0.41 Cent. 
(0.7 F.) above the average. A similar peculiarity can 
be traced to other zones, but on account of the less 
steady climates it is much less marked there than in the 
tropics. A French physicist, Nordmann, has fully con- 
firmed the observations of Koppen. On the other hand, 
Very, an American astronomer, has found that the tem- 
perature in very dry (desert) districts of the tropics 
(near Port Darwin, 12 28' S., and near Alice Springs, 
23 38' S., both in Australia) is higher at sun-spot maxi- 
ma than at minima; but Very was in this research guided 
merely by the records of maximum and minimum ther- 
mometers. From Very's investigation it would appear 
that the solar radiation is really more intense with larger 
sun-spot numbers. 1 This, it must be remarked, is only 
noticeable in exceedingly dry districts in which there is no 
cloud formation worth mentioning. In other districts the 
cloud formation which accompanies sun-spot maxima in- 
terferes with the simplicity of the phenomena. The cool- 
ing effect of the clouds seems in these cases by far to sur- 
pass the direct heating effect of the solar rays, and in this 
manner Koppen's conclusion would become explicable. If 
we could observe the temperatures of the atmospheric 
strata above the clouds, their variation would no doubt 
be in the same degree as that in the desert. 
Finally, we have to note another period in the phe- 

1 According to Memery (Bull. Soc. Astr., March 7, 1906, p. 168) an 
instantaneous rise of temperature is observed immediately when a 
sun-spot is first seen, and the temperature sinks again when the 
sun-spot disappears. 



nomena of the polar lights the so-called tropical month, 
whose length is 27.3 clays. The nature of this period is 
little understood. It is possibly connected with the 


4 ft 

Fig. 42. Curve of magnetic declination at Kew, near Lon- 
don, on November 15 and 1C, 1905. The violent disturb- 
ance of November 15, 9 P.M., corresponds to the maximum 
intensity of the aurora. Compare the following figure 

electric charge of the moon. The peculiarity of this 
period is that it acts in an opposite way in the northern 
and southern hemispheres. When the moon is above 
the horizon, it seems to prevent the formation of polar 
lights; but for this case the difficulties of observation 
caused by the moonlight must, of course, be taken into 

Celsius and Hiorter observed in 1741 that the polar 
lights exercise an influence on the magnetic needle. 
From this circumstance we have drawn the conclusion 
that the polar lights are in some way due to electric dis- 
charges which act upon the magnetic needle. These 
magnetic effects, the disturbances of the otherwise steady 
position of the magnetic needle, are not influenced by 
the light of the sun and moon, and can therefore be 



studied to greater advantage than auroras. We have 
already pointed out that it is only the aurora of the 
radial, streamer type which exerts this magnetic influence 
(compare Figs. 42 and 43). 

These magnetic variations show exactly the same 
periods as the northern lights and the sun-spots. As 
regards, first, the long period of 11.1 years, our observa- 
tions prove that the so-called magnetic disturbances of 
the position of the magnetic needle faithfully reflect the 
variations in the sun-spots. This connection was dis- 
covered in 1852 by Sabine in England, by Wolf in Switzer- 
land, and by Gautier in France. Even the more irregu- 
lar diurnal variations in the magnetic elements are sub- 
jected to a solar period. The magnetic needle points 
in our districts with its north end towards the north not 
exactly, though, being deflected towards the west. This 


/Vo> ,S 

4 a , Ar* 


Fig. 43. Curve of horizontal intensity at Kew on November 
15 and 10, 1905. On November 15 a magnificent aurora 
was observed in Galicia, Germany, France, Norway, Eng- 
land, Ireland, and Nova Scotia, with a maximum at 9 P.M. 
The polar light was unusually brilliant as early as 6 P.M. 

western deviation or declination is greatest soon after 
noon, about one o'clock, and this diurnal change is greater 
in summer than in winter, and the fluctuation of the 



position of the magnetic needle greater in daytime than 
at night-time. It is, therefore, manifest that we have to 
deal with some solar effect. This becomes still more dis- 
tinct when we study the change with reference to the 
daily variation in the number of sun-spots. In the sub- 
joined table the variation in the declination has been 
compiled for Prague for the years 1856 to 1889. Only 
years with maxima and minima of sun-spots and of mag- 
netic variations have been noticed in this table: 

'1&56 I860 1867 1871 1879 1884 1889 

Sun-spot number.. 4. 3 95.7 7.3 139.1 3.4 63.7 6.3 

1856 1859 1867 1871 1878 1883 1889 

Observed ... 5.98 10.36 6.95 11.43 5.65 8.34 5.99 
Calculated... 6.08 10.20 6.22 12.15 6.04 8.76 6.17 

We see that the maxima and minima years of the two 
phenomena very nearly coincide. The accord is so evi- 
dent that we may calculate the diurnal variation as pro- 
portional to the increase in the number of sun-spots. 
This is shown by the two last lines of the table. 

The yearly variation is again exactly the same as that 
of polar lights, as the following table indicates, in which 
the disturbances of magnetic declination, horizontal in- 
tensity, and vertical intensity are compiled for Toronto, 
Canada; for comparison the means of these three magni- 
tudes are added for Greenwich. The unit of this table 
is the- average annual variation: 

Jan. Feb. Mar. April May June July Aog. Sept. Oot. Nor. Dec. 

Declination.. 0.57 0.84 1.11 1.42 0.98 0.53 0.94 1.16 1.62 1.31 0.78 0.76 
Horizontal... 0.56 0.94 0.94 1.5O 0.90 0.36 0.61 0.75 1.71 J.48 098 0.58 
Vertical 0.57 0.74 1.08 1.49 1.12 0.50 0.71 1.08 1.61 1.29 0.75 0.61 


Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 

Means.. 0.93 1.23 1.22 1.09 0.81 0.71 0.81 0.90 1.15 1.18 1.02 0.83 



The daily variation of the disturbances has been an- 
alyzed by Van Bemmelen for the period 1882-1893 and 
for the observatory of Batavia, on Java. The maxi- 
mum occurs there about 1 P.M., and is about 1.86 times 
as great as the average value for the day. The minimum 
of 0.48 occurs at 11 P.M. Between 8 P.M. and 3 A.M. 
the disturbances are almost as rare as about 11 o'clock 
at night. 

The variation is greatest with that declination which 
has its maximum of 3.26 at 12 M., and its minimum of 
0.14 at 11 P.M. 

The period of almost 26 days first investigated by Horn- 
stein has also been refound in the magnetic variations 
and disturbances by Broun, Liznar, and C. A. Miiller. 
It must be added, however, that Schuster does not con- 
sider these data as in any way conclusive. 

The moon has also a slight influence upon the magnetic 
needle, as Kreil proved as early as 1841. The effect is in 
a different sign in the northern and southern hemi- 
spheres, and may be likened to a tidal phenomenon. 

The ultra-violet rays of the sun are strongly absorbed 
by the atmosphere, and they cause an ionization of the 
molecules of the air. This ionization is, on the whole, 
more marked at higher altitudes. The ascending air 
currents carry with them water vapor which is condensed 
on the ions, particularly on the negative ions. In this 
way most clouds become negatively charged; this inter- 
esting fact i. e., that they are more frequently charged 
with negative than with positive electricity was first 
proved by Franklin in his kite experiments. When the 
rain-drops have fallen, the air above remains positively 
charged; this has been observed during balloon ascen- 
sions. The clouds which are formed at high levels are 
most strongly charged; for this reason thunder-storms 



over land occur mostly in the summer-time. The thun- 
der-storms also show the 26-day period, as Bezold has 
proved for southern Germany, and Ekholm and myself 
have shown for Sweden. 

A vast amount of material concerning these questions 
and magnetic phenomena in particular has been col- 
lected by the various meteorological observatories and 
is awaiting analysis. 

Although some observers like Sidgreaves question the 
correlation of sun-spots and polar lights or magnetic 
disturbances, because strong spots have been seen on 
the disk of the sun without any magnetic disturbances 
having been noticed, yet the view predominates that the 
magnetic disturbances are caused by sun-spots when the 
sun-spots cross the central meridian of the sun which is 
opposite the earth. Thus Maunder observed a magnetic 
storm and a northern light succeeding the passage of a 
large sun-spot through the central solar meridian on the 
8th to the 10th of September, 1898. The magnetic effect 
attained its maximum about twenty-one hours after the 
passage through the meridian. 

Similarly Ricco established in ten instances, in which 
exact determination was possible, a time interval of 
45.5 hours on an average between the meridian passage 
of a spot and the maximum magnetic effect. Ricco 
also submitted to an analysis the data which Ellis had 
collected and which Maunder had investigated. He 
found for these instances, on an average, almost exactly 
the same numbers, the time interval being 42.5 hours. 
That would correspond to a mean velocity of the solar 
dust of from 910 to 980 km. per second. On the other 
hand, we have no difficulty at all in calculating the time 
which a spherule of a diameter of 0.00016 mm. (those 
particles travel fastest) and of the specific gravity of 



water would need in order to reach the earth, under the 
influence of solar gravitation and of a mechanical radia- 
tion pressure 2.5 times as large from the outside of the 
sun. The time found, 56.1 hours, corresponds to a mean 
velocity of 740 km. per second. In order that the solar 
dust may move with the velocity calculated by Ricco, its 
specific gravity should be less than 1 viz., 0.66 and 0.57. 
This value looks by no means improbable, when we as- 
sume that the spherules consist of hydro-carbons satu- 
rated with hydrogen, helium, arid other noble gases. We 
should also arrive at larger velocities for the solar dust, 
as has already been pointed out with regard to the tails 
of comets, when we presume that the particles consist 
of felted marguerites of carbon or silicates, or of iron 
materials which we regard as the main constituents of 

It should, perhaps, be mentioned that the most in- 
tense spectrum line of the polar lights has been found 
to be characteristic of the noble gas krypton. As this 
gas is found only in very small quantities in our atmos- 
phere, it does not appear improbable that it has been 
carried to us together with the solar dust, and that its 
spectrum becomes perceptible during the discharge phe- 
nomena. The other spectrum lines of the polar lights 
belong to the spectra of nitrogen, argon, and of the 
other noble gases. The volumes of noble gases which are 
brought into our terrestrial atmosphere in this manner 
would in any case be exceedingly small. 

The electrical phenomena of our terrestrial atmos- 
phere indirectly possess considerable importance for or- 
ganic life and, consequently, for human beings. By the 
electrical discharges part of the nitrogen is made to com- 
bine with the oxygen and hydrogen (liberated by the 
electric decomposition of water vapor) of our air, and it 



thus forms the ammonia compounds, as well as the nitrates 
and nitrites, which are so essential to vegetable growth. 
The ammonia compounds which play a most important 
part in the temperate zones appear especially to be formed 
by the so-called silent discharges which we connect with 
auroras. The oxygen compounds of nitrogen, on the 
other hand, seem to be chiefly the products of the violent 
thunder-storms of the tropics. The rains carry these 
compounds down into the soil, where they fertilize the 

The supply of nitrogen thus fixed amounts in the 
course of a year to about 1.25 gramme per square metre 
in Europe, and to almost fourfold that figure in the 
tropics. If we accept three grammes as the average num- 
ber for the whole firm land of the earth, that would mean 
3 tons per square kilometre, and about 400 million tons 
per year for the whole firm land of 136 million square 
kilometres. A very small portion of this fixed nitrogen, 
possibly one-twentieth, falls on cultivated soil; a larger 
portion will help to stimulate plant growth in the forests 
and on the steppes. We may mention, for comparison, 
that the nitrogen contained in the saltpetre which the 
mines of Chili yield to us has risen from 50,000 tons in 
1880 to 120,000 tons in 1890, to 210,000 tons in 1900, 
and to 260,000 tons in 1905. The nitrogen produced in 
the shape of ammonium salts (sulphate) by the gas-works 
of Europe amounts to about one-quarter of the last- 
mentioned figure. To this figure we have, of course, to 
add the production of coal-gas ammonia in the United 
States and elsewhere. Yet even allowing for this item, 
we shall find that the artificial supply of combined nitro- 
gen on the earth does not represent more than about one- 
thousandth of the natural supply. 

As the nitrogen contents of the air may be estimated 



at 3980 billion tons, we recognize that only one part in 
three millions of the nitrogen of the atmosphere is every 
year fixed by electric discharges, presuming that the 
nitrogen supply to the sea is as great per square kilo- 
metre as to the land. The nitrogen thus fixed benefits 
the plants of the sea and of the land, and passes back into 
the atmosphere or into the water during the life of the 
plants or during their decay. Water absorbs some nitro- 
gen, and equilibrium between the nitrogen contents of 
the atmosphere and of the sea is thus maintained. Hence 
we need not fear any noteworthy depletion of the nitro- 
gen contents of the air. This conclusion is in accord with 
the fact that no notable accumulation of fixed nitrogen 
appears to have taken place in the solid and liquid con- 
stituents of the earth. 

The reader may remember (compare page 57) that 
during the annual cycle of vegetation not less than one- 
fiftieth of the atmospheric contents in the carbon dioxide 
is absorbed. Since oxygen is formed from this carbon 
dioxide, and since the air contains about seven hundred 
times as many parts per volume of oxygen as of carbon 
dioxide, the exchange of atmospheric oxygen is about 
one part in thirty-five thousand. In other words, the 
oxygen of the air participates about one hundred times 
more energetically in the processes of vegetation than the 
nitrogen, and this is in accordance with the general high 
chemical activity of oxygen. 

Before we close this chapter we will briefly refer to 
the peculiar phenomenon known as the Zodiacal Light, 
which can be seen in the tropics almost any clear night 
for a few hours after or before sunset or sunrise. In our 
latitudes the light is rarely visible, and is best seen about 
the periods of the vernal and autumnal equinoxes. The 
phenomenon is generally described as a luminous cone 



whose basis lies on the horizon, and whose middle line 
coincides with the zodiac. Hence the name. Accord- 
ing to Wright and Liais, its spectrum is continuous. It 
is stated that the Zodiacal Light is as strong in the tropics 
as that of the Milky Way. 

There can be no doubt that this glow is due to dust 
particles illuminated by the sun. It has therefore been 
assumed that this dust is floating about the sun in a ring, 

Fig. 44. Zodiacal Light in the tropics 


and that it represents the rest of that primeval nebula 
out of which the solar system has been condensed, ac- 
cording to the theory of Kant and Laplace (compare 
Chapter VII.). Sometimes a fairly luminous band seems 
to shoot out from the apex of the cone of the Zodiacal Light 
and to cross the sky in the plane of the ecliptic. In that 
part of the sky which is just opposite the sun this band 
expands to a larger, diffused, not well-defined spot of 
light covering about 12 of arc in latitude and 90 in lon- 
gitude. This luminescence is called the counter -glow 
(Gegenschein), and was first described by Pezenas in 

The most probable view concerning the nature of this 
counter-glow is that it is caused by small particles of 
meteorites or dust which fall towards the sun. Like the 
position of the corona of the aurora, the position of this 
counter-glow seems to be an effect of perspective; the 
orbits of the little particles are directed towards the sun, 
and they therefore appear to radiate from a point op- 
posite to it. 

We know very little about this phenomenon. Even 
the position of the Zodiacal Light along the zodiac which 
has given rise to its name has been questioned, and it 
would appear from recent investigations that the glow 
is situated in the plane of the solar equator. However 
that may be, the view is very generally held that the glow 
is due to particles which come from the sun or enter 
into it. We have already adduced arguments to prove 
that the mass of solar dust cannot be unimportant; this 
dust may therefore be the cause of the phenomenon 
which we have just been discussing. 



WE have seen that the sun is dissipating and wasting 
almost inconceivable amounts of heat every year: 3.8. 
10 33 gramme-calories, corresponding to 2 gramme-calories 
for each gramme of its mass. We have also obtained an 
idea as to how the enormous storage of heat energy in 
the sun may endure this loss for ages. Finally, however, 
the time must come wken the sun will cool down and 
when it will cover itself with a solid crust, as the earth 
and the other planets so far, probably, in a gaseous 
state have done long since or will do before long. No 
living being will be able to watch this extinction of the 
sun despairingly from one of the wandering planets; 
for, in spite of all our inventions, all life will long before 
have ceased on the satellites of the sun for want of heat 
and light. 

The further development of the cold sun will recall the 
actual progress of our earth, except in so far as the sun 
will have no life-spending, central source of light and 
heat near it. In the beginning the thin, solid crust will 
again and again be burst by gases, and streams of lava 
will rush out from the interior of the sun. After a while 
these powerful discharges will stop, the lava will freeze, 
and the fragments will close up more firmly than before. 
Only on some of the old fissures volcanoes will rise and 
allow the gases to escape from the interior water vapor 



and, to a less extent, carbonic acid, liberated by the 

Then water will be condensed. Oceans will flood the 
sun, and for a short period it will resemble the earth in its 
present condition, though with the one important differ- 
ence. The extinct sun, unlike our earth, will not receive 
life-giving heat from the outside, excepting the small 
amount of radiation from universal space and the heat 
generated by the fall of meteorites. The temperature fall 
will therefore be rapid, and the vanishing clouds of the at- 
tenuated atmosphere will not long check radiation. The 
ocean will become covered with a crust of ice. Then the 
carbonic acid will commence to condense, and will be pre- 
cipitated as a light snow in the solar atmosphere. Fi- 
nally, at a temperature of about 200 Cent., the gases of 
the atmosphere will be condensed, and new oceans, now 
principally of nitrogen, will be produced. Let the tem- 
perature sink another 20, and the energy of the inrush- 
ing meteorites will just suffice to balance a further loss 
of heat by radiation. The solar atmosphere will then 
consist essentially of helium and hydrogen the two 
gases which are most difficult to condense and of some 

In this stage the heat loss of the sun will be almost 
imperceptible. Owing to the low thermal conductive 
power of the earth's crust, there escapes through each 
square mile of this crust scarcely one-thousand-millionth 
part of the heat which the sun is radiating from an equal 
area of its surface. In future days, when the solar crust 
will have attained a thickness of 60 km. (40 miles), its 
loss of heat will be diminished to the same degree. The 
temperature on the surface of the sun may then still be 
some 50 or 60 above absolute zero, and volcanic erup- 
tions will raise the temperature only for short periods 



and over small areas. Yet in the interior the tempera- 
ture will still be at nearly the same actual intensity, some- 
thing like several million degrees, and the compounds of 
infinite explosive energy will be stored up there as to- 
day. Like an immense dynamite magazine, the dark sun 
will float about in universal space without wasting much 
of its energy in the course of billions of years. Immutable, 
like a spore, it will retain its immense store of force until 
it is awakened by external forces into a new span of life 
similar to the old life. A slow shrinkage of the surface, 
due to the progressive loss of heat of the core and to the 
consequent contraction, will in the meanwhile have cov- 
ered the sun with the wrinkles of old age. 

Let us suppose that the crusts of the sun and the earth 
have the same thermal conductivity namely, that of 
granite. According to Homen, a slab of granite one 
centimetre in thickness, whose two surfaces are at a tem- 
perature difference of 1 Cent., will permit 0.582 calorie 
to pass per minute per square centimetre of surface. 
By analogy, the earth's crust, with an increase of tem- 
perature of 30 per kilometre, as we penetrate inward, 
would allow 1.75 .10~ 4 calorie to pass per minute and 
per square centimetre (this is -g-gVo f the mean heat sup- 
ply of the earth, 0.625 calorie per minute per square centi- 
metre); while the sun, with a crust of the same thick- 
ness as the earth, but with a diameter 108.6 times larger, 
would lose 3.3 times more heat per minute than the earth 
receives from it at the present time. At present the sun 
loses 2260 million times more heat than the earth re- 
ceives; consequently, the loss of heat would be reduced 
t 6T67ffo0T<ra~(T f the present amount. If the thickness 
of the solar crust amounted to yj-^ of the solar radius 
that is to say, to the same fraction that the thickness of 
the earth's crust represents of the terrestrial radius the 



sun would in 74,500 million years not lose any more heat 
than it does now in a single year. This number has to 
be diminished, on account of the colder surface which 
the sun would have by that time, to about 60,000 million 
years. Considering that the mean temperature of the 
sun may be as high as 5 million degrees Celsius, the cool- 
ing down to the freezing-point of water might occupy 
150,000 billion years, assuming that its mean specific 
heat is as great as that of water. During this time the 
crust of the sun would increase in thickness and the cool- 
ing would, of course, proceed at a decreasing rate. In 
any case, the total loss of energy during a period of a 
thousand billion years could, under these circumstances, 
only constitute a very small fraction of the total stored 

When an extinct star moves forward through infinite 
spaces of time, it will ultimately meet another luminous 
or likewise extinct star. The probability of such a col- 
lision is proportional to the angle under which the star ap- 
pears which, though very small, is not of zero magnitude 
and to the velocity of the sun. The probability is in- 
creased by the deflection which these celestial bodies will 
undergo in their orbits on approaching each other. Our 
nearest neighbors in the stellar universe are so far removed 
from us that light, the light of our sun, requires, on an av- 
erage, perhaps ten years to reach them. In order that the 
sun, with its actual dimensions and its actual velocity in 
space 20 km. (13 miles) per second should collide with 
another star of similar kind, we should require something 
like a hundred thousand billion years. Suppose that 
there are a hundred times more extinct than luminous 
stars an assumption which is not unjustifiable the 
probable interval up to the next collision may be some- 
thing like a thousand billion years. The time during 

ii 151 


which the sun would be luminous would represent per- 
haps one-hundredth of this that is to say, ten billion 
years. This conclusion does not look unreasonable. For 
life has only been existing on the earth for about a thou- 
sand million years, and this age represents only a small 
fraction of the time during which the sun has emitted 
light and will continue to emit light. The probability of 
a collision between the sun and a nebula is, of course, 
much greater; for the nebulae extend over very large 
spaces. In such a case, however, we need not apprehend 
any more serious consequences than result when a comet 
is passing through the corona of the sun. Owing to the 
very small amount of matter in the corona, we have not 
perceived any noteworthy effects in these instances. 
Nevertheless, the entrance of the sun into a nebula would 
increase the chance of a collision with another sun; for 
we shall see below that dark and luminous celestial 
bodies appear to be aggregated in the nebulae. 

From time to time we see new stars suddenly flash up 
in the sky, rapidly decrease in splendor again, to become 
extinguished or, at any rate, to dwindle down to faint 
visibility once more. The most remarkable of these ex- 
ceedingly interesting events occurred in February, 1901, 
when a star of the first magnitude appeared in the con- 
stellation of Perseus. This star was discovered by An- 
derson, a Scotchman, on the morning of February 22, 
1901. It was then a star of the third magnitude. 1 On 
a photograph which had been taken only twenty-eight 

1 Stars are classified in magnitude, the order being such that the 
most luminous stars have the lowest numbers. A star of the first 
magnitude is 2.52 times brighter than a star of the second magnitude; 
this, again, 2.52 times brighter than a star of the third magnitude, 
and so on. All this from the point of view of an observer on the 



hours previous to the discovery of this star, the star was 
not visible at all, although the plate marked stars down 
to the twelfth magnitude. The light intensity of this 
new star would hence appear to have increased more 
than five-thousand-fold within that short space of time. 
On February 23d the new star surpassed all other stars 
except Sirius in intensity. By February 25th it was of 
the first magnitude, by February 27th of the second, by 
March 6th of the third, and by March 18th of the fourth 
magnitude. Then its brightness began to fluctuate peri- 
odically up to June 22d, with a period first of three, then 
of five days, while the average light intensity decreased. 
By June 23d it was of the sixth magnitude. The light 
intensity diminished then more uniformly. By October, 
1901, it was a star of the seventh magnitude ; by February, 
1901, of the eighth magnitude; by July, 1902, of the 
ninth magnitude; by December, 1902, of the tenth mag- 
nitude; and since then it has gradually dwindled to the 
twelfth magnitude. When this star was at its highest 
intensity it shone with a bluish- white light. The shade 
then changed into yellow, and by the beginning of March, 
1901, into reddish. During its periodical fluctuations the 
hue was whitish yellow at its maximum and reddish at 
its minimum intensity. Since then the color has grad- 
ually passed into pure white. 

The spectrum of this star shows the greatest similarity 
to that of the new star in the constellation Auriga (Nova 
Aurigae) of the year 1892 (Fig. 45). 

On the whole, it is characteristic of new stars that the 
spectrum lines appear double dark on the violet arid 
bright on the red side. In the spectrum of Nova Aurigse 
this peculiarity is, among others, striking in the three hy- 
drogen lines C, F, and H, in the sodium line, in the nebula 

lines, and also in the magnesium line. In the spectrum 




CSO (60 GM) 

Fig. 45. Spectrum of Nova Aurigae, 1892 

of Nova Persei the displacement of the hydrogen lines 
towards the violet is so great that, according to Doppler's 
principle, 1 the hydrogen gas which absorbed the light 
must have been moving towards us with a velocity of 
70Q or more kilometres (450 miles) per second. Some 
calcium lines show a similar displacement, which is less 
noticeable in the case of the other metals. This would 

1 When, standing on a station platform, we watch an express train 
rushing through the station, the pitch of the engine whistle seems to 
become higher as long as the train is approaching us, and deeper again 
when the train is moving away from us. The pitch of a note depends 
upon the number of oscillations which our ear receives per second. 
Now, when the train is fast approaching us, more vibrations are sent 
into our ear than when the train is at a stand-still, and the pitch, 
therefore, appears to become higher. The same reasoning holds for 
light waves, of which Doppler, of Prague (Bohemia), was in fact think- 
ing when first announcing his principle in 1842. The wave-length of 
a particular color of the spectrum is fixed with the aid of some Fraun- 
hofer line characteristic of a certain metal. If we compare the spec- 
trum of a star and the spectrum of a glowing metal, photographed 
on the same plate, the stellar lines will appear shifted towards the 
violet end (violet light is produced by nearly twice as many vibrations 
of the ether per second as red light) when the star is moving towards 
us in the line of sight. This principle has successfully been applied 
by Huggins, H. C. Vogel, and others, for determining the motion of a 
star in our line of sight. When a star is revolving about its own 
axis, the equatorial belt will seem to come nearer to us (or to recede 
from us), while the polar regions will seem to be at a stand-still; the 
lines will then appear oblique (not vertical). In this way Keeler 
proved that the rings of Saturn consists of swarms of meteorites mov- 
ing at different velocities in the different rings. H. B. 



appear to indicate that relatively cold masses of gas are 
issuing from the stars and streaming with enormous " 
velocities towards the earth. The luminous parts of the 
stars w r ere either at a stand-still or they were moving away 
from us. The simplest explanation of these phenomena 
would be that a star when flashing up by virtue of its 
high temperature and high pressure shows enhanced 
(widened) spectral lines, whose violet portion is absorbed 
by the strongly cooled masses of gas which are moving 
towards us and are cooled by their own strong expansion. 
These gases stream, of course, in all directions from the 
star, but we only become aware of those gases which ab- 
sorb the light of the stars that is to say, those situated 
between the star and the earth, and streaming in our di- 

Gradually the light of the metallic lines and of the con- 
tinuous spectrum on which they were superposed began 
to fade, first in the violet, while the hydrogen lines and 
nebular lines still remained distinct ; like other new stars, 
this star showed, after a while, the nebular spectrum. 
This interesting fact was first noticed by H. C. Vogel in 
the new star in the Swan (Nova Cygni, 1876). The star 
P in the Swan, which flashed up in the year 1600, still 
offers us a spectrum which indicates the emission of hydro- 
gen gas. It is not impossible that this "new" star has 
not yet reached its equilibrium, and is still continuing to 
emit cold streams of gases. Insignificant quantities of 
gas suffice for the formation of an absorption spectrum; 
thus the emission of gas might continue for long periods 
without exhausting the supply. 

We have already mentioned (page 116) the peculiar 
clouds of light which were observed around Nova Persei. 
Two annular clouds moved away from this star with veloc- 
ities of 1.4 and 2.8 seconds of arc per day between March 



29, 1901, and February, 1902. If we calculate backward 
from these dates the time which must have elapsed since 
those gases left the star, we arrive at the date of the 
week February 8 to 16, 1901 in satisfactory agreement 
with the period of greatest luminosity of the star of 
February 23d. It would, therefore, appear that these 
emanations came from the star and were ejected by the 
radiation pressure. Their light did not mark any no- 
ticeable polarization, and could not be reflected light for 
this reason. We may suppose that the dust particles 
discharged their electric charges, and that the gases be- 
came thereby luminous. 

In this case we were evidently witnesses of the grand 
finale of the independent existence of a celestial body 
by collision with some other body of equal kind. The 
two colliding bodies were both dark, or they emitted so 
little light that their combined light intensities did not 
equal that of a star of the twelfth magnitude. As, now, 
their splendor after the collision was greater than that 
of a star of the first magnitude, although their distance 
has been estimated to be at least 120 light years, 1 their 
radiation intensity must have exceeded that of the sun 
several thousand times. Under these circumstances the 
mechanical radiation pressure must also have been many 
times more powerful than on the surface of the sun, and 
the masses of dust which were ejected by the new star 
must have possessed a velocity very much greater than 
that of solar dust. Yet this velocity i$ust have been 
smaller than that of light, which, indeed, the effect of 
the radiation pressure can never equal. 

It is not difficult to picture to ourselves the enormous 
violence with which this " collision" must have taken 

1 One light year corresponds to 9.5 billions kilometres, and it is 
the distance which the light traverses in the course of a year. 



place. A strange body for instance, a meteorite 
which rushes from the infinite universe into the sun has 
at its collision a velocity of 600 km. (400 miles) per 
second, and the velocity of the two colliding suns must 
have been of approximately that order. The impact 
will in general be oblique, and, although part of the 
energy will of course be transformed into heat, the rest 

Fig. 46. Diagram indicating the consequences of a collision 
between two extinct suns, A and B ' moving ' in the direc- 
tion of the straight arrows. A rapid rotation in the di- 
rection of the curved arrows results, and two powerful 
streamers are ejected by A B, the explosive substances from 
the deeper strata of A and B being brought up to the sur- 
face by the collision 

of the kinetic energy must have produced a rotational 
velocity of hundreds of kilometres per second. By com- 
parison with this number the actual circumferential speed 
of the sun, about 2 km. (1} miles) per second on the 
equator, would vanish altogether; and the difference is 
still more striking for the earth, with its 0.465 km. per 
second at the equator. We shall, therefore, not commit 

an error of any consequence if we presume the two bodies 



to have been practically devoid of circumferential speeds 
before their collision. At the collision, matter will have 
been ejected from both these celestial bodies at right 
angles to the relative directions of their motion in two 
powerful torrents, which would be situated in the plane 
in which the two bodies were approaching each other 
(compare Fig. 46). The rotational speed of the double 
star, which will be diminished by this ejection of matter, 
will have contributed to increase the energy of ejection. 
We remember, now, that when matter is brought up 
from the interior to the surface of the sun it will behave 
like an explosive of enormous power. The ejected gases 
will be hurled in terrific flight about the rapidly revolving 
central portions. We obtain an idea (though a very im- 
perfect one) of these features when we look at a revolving 
pinwheel in a fireworks display. Two pinwheels have 
been attached to the ends of a diameter and belch forth 
fire in radial lines. The farther removed from the wheel, 
the smaller will be the actual velocity and also the angu- 
lar velocity of these torrents of fire. Similarly with 
the star. The streams are rapidly cooled, owing to the 
rapid expansion of the gas. They will also contain fine 
dust, largely consisting of carbon, probably, which had 
been bound by the explosive materials. The clouds of 
fine dust will obscure the new star more and more, and 
will gradually change its white brilliancy into yellow and 
reddish, because the fine dust weakens blue-and-green 
rays more than it does yellow-and-red rays. At first the 
clouds were so near to the the star that they possessed a 
high angular velocity of their own; they then appeared to 
surround the star completely. But after March 22, 1901, 
the outer particles of the streams attained greater dis- 
tances and assumed longer periods of revolution (six 
days); the star then became more obscured when the 



extreme dust clouds of the streams covering it happened 
to get between us and the star. As the streams of par- 
ticles were moving farther away, their rotational periods 
increased gradually to ten days. The star, therefore, 
became periodical with a slowly growing length of 
period, and its glow turned more reddish at its min- 
imum than at its maximum of intensity. At the same 
time, the absorptive power of the marginal particles 
decreased, partly owing to their increasing expansion, 
partly because the dust was slowly aggregating to coarser 

Fig. 47. Spiral nebula in the Canes Venatici. Messier 51. 
Taken at the Yerkes Observatory on June 3, 1902. Scale, 
1 mm. =13. 2 sec. of arc 



particles ; possibly, also, because the finest particles were 
being driven away by the radiation pressure. The sift- 
ing influence which the dust exercised upon the light, 
and owing to which the red-and-yellow rays were more 
readily transmitted than the blue-and-green, gradually 
became impaired ; hence the color of the light Burned 
more gray, and after a certain time the star appeared 
once more of a whitish hue. This white color indicates 
that the star must still have a very high temperature. 
By the continued ejection of dust-charged masses of 
gas, probably with gradually decreasing violence, the 
light intensity of the star must slowly diminish (as seen 
from the earth) and the distribution of the layers of 
dust around the luminous core will more and more be- 
come uniform. How violent the explosion must have 
been, we recognize from the observation that the first 
ejected masses of hydrogen rushed out with an apparent 
velocity of at least 700 km. per second. This velocity is 
of the same order as that of the most remarkable promi- 
nences of the sun. 

It will be admitted that these arguments present us 
with a faithful simile even of the details of the observed 
course of events, and it is therefore highly probable that 
our view is in the main correct. But what has mean- 
while become of the new star? Spectrum analysis tells us 
that it has been converted into a stellar nebula like other 
new stars. The continuous light of the central body has 
more and more been weakened by the surrounding masses 
of dust. By the radiation pressure these masses are 
driven towards the outer particles of the surrounding 
gaseous envelope consisting principally of hydrogen, 
helium, and " nebular matter." There the dust dis- 
charges its negative electricity, and thus calls forth a 
luminescence which equals that of the nebulae. 


Fig. 48. Spiral nebula in the Triangle. Messier 33. Taken at the 
Yerkes Observatory on September 4 and 6, 1902. Scale, 1 nun.= 
30.7 sec. of arc 


We have to consider next that owing to the incredibly 
rapid rotation, the central main mass of the two stars 
will, in its outer portions, be exposed to centrifugal forces 
of extraordinary intensity, and will therefore become 
flattened out to a large revolving disk. 1 As the pressure 
in the outer portions will be relatively small, the density 
of the gases will likewise be diminished there. The en- 
ergetic expansion and, more still, the great heat radiation 
will lower the temperature at a rapid rate. We have 
thus to deal with a central body whose inner portion will 
possess a high density, and which will resemble the mass 
of the sun, while the outer portion will be attenuated and 
nebular. Distributed about the central body we shall 
find the rest of the two streams of gases which were eject- 
ed immediately after the violent collision between the 
two celestial bodies. A not inconsiderable portion of 
the matter of these spirally arranged outer parts will 
probably travel farther away into infinite space, finally 
to join some other celestial body or to form parts of the 
great irregular spots of nebular matter which are col- 
lected around the star clusters. Another portion, not 
able to leave the central body, will remain in circular 
movement about it. In consequence of this circular 
movement, which will be extremely slow, the outlines of 
the two spirals will gradually become obliterated, and 
the spirals will themselves more and more assume the 
shape of nebular rings about the central mass. 

This spiral form (Figs. 47 and 48) of the outer portions 

1 A. Hitter has calculated that when two suns of equal size collide 
with one another from an infinite distance, the energy of the collision 
is not more than sufficient to enlarge the volume of the suns to four 
times the previous amount. The largest portion of the mass will 
therefore probably remain in the centre, and it will 'only be masses 
of light gases which will be ejected. 


Fig. 49. The great nebula in Andromeda. Taken at the Yerkes 
Observatory on September 18, 1901. Scale, 1 mm. = 54.6 sec. of arc 


of the nebulse has for a long time excited the greatest 
attention. In almost all the investigated instances it 
has been observed that two spiral branches are coiling 
about the central body. This would indicate that the 
matter is in a revolving movement about the cenjtral axis 
of the spiral, and that it has streamed away from the axis 
in two opposite directions. Sometimes the matter ap- 
pears arranged as in a coil; of this type the great nebula 
of Andromeda is the best-known example (Fig. 49). A 
closer inspection of this nebula with more powerful in- 
struments indicates, however, that it is also spiral and 
that it appears coiled, because we are looking at it from 

Fig. 50. Ring-shaped nebula in Lyra. Taken at 
the Yerkes Observatory 

the side. The late famous American astronomer Keeler, 
who has studied these nebulae with greater success than 
any one else, has catalogued a great many of them in all 
the divisions of the heavens which were accessible to his 



instruments, and he has found that these formations are 
predominatingly of a spiral nature. 

Some nebulae, like the so-called planetary nebulae, 
offer rather the appearance of luminous spheres. We 

Fig. 51. Central portion of the great nebula in Orion. Taken at 
the Yerkes Observatory. Scale, 1 mm. =12 sec. of arc 

may assume in these cases that the explosions were less 
violent, and that the spirals, therefore, are situated so 
closely together that they seem to merge into one an- 
other. Possibly the inequalities in their development 
have become equalized in the course of time. A few 
nebuke are ring-shaped, as the well-known nebula of 
Lyra (see Fig. 50). These rings may, again, have been 



formed out of spiral nebulse, and the spirals may have 
gradually been obliterated by rotation, while the central 
nebulous matter may have been concentrated on the 
planets travelling round the central star. Schaeberle, an 
eminent American astronomer, has discovered traces of 
spiral shape also in the Lyra nebula. 

Another kind of nebula is the ordinary nebula of 
vast extension and irregular shape, evidently formed out 
of most extremely attenuated matter; well-known char- 
acteristic examples are found in Orion, about the Pleiades, 
and in the Swan (Figs. 51, 52, and 53). In these nebula? 
portions of a spiral structure have likewise often been dis- 

We have said that the collision between two celestial 
bodies would result in the formation of a spiral with two 
wings. If the impact is such that the two centres of the 
celestial bodies move straight towards each other, a disk 
will arise, and not a spiral ; or if one star is much smaller 
than the other, possibly a cone, because the gases will 
uniformly be spread in all directions about the line of 
impact. A perfectly central impact is obviously very 
rare; but there may be cases which approach this limit- 
ing condition more or less, especially when the relative 
velocity of the two bodies is small. By slow diffusion 
a feebly developed spiral may also be converted into a 
rotating disklike structure. The extension of these 
nebular structures will depend upon the ratio between 
the mass of the system and the velocity of ejection of 
the gases. If, for example, two extinct suns of nearly 
equal dimensions and mass, like our sun, should collide, 
some gas masses would travel into infinite space, being 
hurled out with a velocity of more than 900 km. (550 
miles) per second; while other particles, moving at a 
slower rate, would remain in the neighborhood of the 


Fig. 52. Nebular striae in the stars of the Pleiades. Taken at the 
Yerkes Observatory on October 19, 1901. Scale, 1 mm. =42.2 sec. 
of arc 


central body. The nearer to that body, the smaller 
was their velocity. From their position they might fall 
back into the central body, to be reiucorporated in it, 
if two circumstances did not prevent this. The one cir- 
cumstance is the enormous radiation pressure of the 
glowing central mass. That pressure keeps masses of 
dust particles floating, which by friction will carry the 
surrounding masses of gas with them. Owing to the 
absorption of the radiation by the dust particles, only 
the finer particles will be supported farther outside, and 
at the extreme margin of the nebula even the very finest 
dust will no longer be maintained in suspension by the 
greatly weakened radiation pressure. Thus we arrive at 
an outer limit for the nebula. The other circumstance is 
the violent rotation which is set up by the impact of the 
central bodies. The rotation and the centrifugal forces 
will produce a disk-shaped expansion of the whole cen- 
tral mass. Owing to molecular collisions and to tidal 
effects, the angular velocity will in the denser portions 
tend to become uniform, so that the whole will rotate 
like a flattened-out ball filled with gas, and the spiral 
structure will gradually disappear in those parts. In 
the more remote particles the velocity will only increase 
to such an extent as to equal that of a planet moving 
at the same distance that is to say, the gravitation tow- 
ards the central body will be balanced by the centrifugal 
force, and at the very greatest distances the molecular 
bombardments, as well as gravitation towards the centre, 
will become so insignificant that any masses collected 
there will retain their shape for an almost unlimited space 
of time. 

In the centre of this system the main bulk of the 
matter would be concentrated in a sun of extreme 
brightness, whose light intensity would, however, owing 


Fig. 53. Nebular striae in the Swan. New General Catalogue, 6992. 
Taken at the Yerkes Observatory on October 5, 1901. Scale, 1 mm. 
=41 sec. of arc 


to strong radiation, diminish with comparative rapid- 

Such an extensive nebular system, in which gravitation, 
on account of the enormous distances, would act feebly 
and very slowly, would yet, in spite of the extraordinary 
attenuation of matter in its outer portions, and just on 
account of its vast extension, be able to stop the move- 
ment of the particles of dust penetrating into it. If the 
gases of the nebula are not to escape into space, notwith- 
standing the infinitesimal gravitation, their molecules 
must be assumed to be almost at a stand-still, and their 
temperature must not rise by more than 50 or 60 Cent, 
above absolute zero. At such low temperatures the so- 
called adsorption plays an enormously important part 
(Dewar). The small dust particles form centres about 
which the gases are condensed to a remarkable degree. 
The extremely low density of these gases does not pre- 
vent their condensation ; for the adsorption phenomenon 
follows a law according to which the mass of condensed 
gas will only be reduced by about one-tenth when the 
density of the surrounding gas has been decreased by 
one-ten-thousandth. The mass of dust particles or dust 
grains will thus be augmented, and when they collide 
they will be cemented together by the semi-liquid films 
condensed upon them. There must, hence, be a rela- 
tively energetic formation of meteorites in the nebula?, 
and especially in their interiors. Then stars and their 
satellites, migrating through space, will stray into these 
swarms of gases and meteorites within the nebulae. The 
larger and more rapidly moving celestial bodies will 
crush through this relatively less dense matter; but 
thousands of years may yet be occupied in their passing 
through nebulae of vast dimensions. 

An extraordinarily interesting photograph obtained by 






the celebrated Professor Max Wolf, of Heidelberg, shows 
us a part of the nebula in the Swan into which a star has 
penetrated from outside. The intruder has collected 
about it the nebulous matter it met on its way, and has 
thus left an empty channel behind it marking its track. 
Similar spots of vast extent, relatively devoid of nebulous 
matter, occur very frequently in the irregular nebulse, 
they are frequently called "fissures," or by the specifi- 

Fig. 55. Great nebula near Rho, in Ophiuchus. Photograph by 
E. E. Barnard, Lick Observatory. There are several empty spots 
and rifts near the larger stars of the nebula 



cally English term "rifts," because they have generally 
a long-drawn-out appearance. The presumption that 
these rit'ts represent the tracks of large celestial bodies 

Fig. 56. Star cluster in Hercules. Messier 13. Taken at the 
Yerkes Observatory. Scale, 1 mm. = 9.22 sec. of arc 

which have cut their way through widely expanded 
nebular masses (Fig. 54) has been entertained for a long 

The smaller and more slowly moving immigrants, on 
the other hand, are stopped by the particles of the 



nebulae. We therefore see the stars more sparsely dis- 
tributed in the immediate neighborhood of the nebulae, 
while in the nebulae themselves they appear more densely 
crowded. This fact had struck Herschel in his observa- 
tions of nebulae; in recent days it has been investigated 
by Courvoisier and M. Wolf. In this way several centres 
of attraction are created in a nebula; they condense the 
gases surrounding the nebula, and catch, so to say, any 
stray meteorites and collect them especially in the inner 
portions of the nebula. We frequently observe, further, 
how the nebular matter appears attenuated at a certain 
distance from the luminous bright stars (compare Figs. 
52 and 55). Finally, the nebulae change into star clusters 
which still retain the characteristic shapes of the nebulae; 
of these the spiral is the most usual, while we also meet 
with conical shapes, originating from conical nebulae, and 
spherical shapes (compare Figs. 56, 57, and 58). 

This is, broadly, the type of evolution through which 
Herschel, relying upon his observations, presumed a 
nebula to pass. He was, however, under the impression 
that the nebulous matter would directly be condensed 
into star clusters without the aid of strange celestial 

It has been known since the most ancient times, and has 
been confirmed by the observations of Herschel and others 
in a most convincing manner, that the stars are strongly 
concentrated about the middle line of the Milky Way. 
It is not improbable that there was originally a nebula 
of enormous dimensions in the plane of the Milky Way, 
produced possibly by the collision of two such giant 
suns as Arcturus. This gigantic nebula has gathered up 
the smaller migrating celestial bodies which, in their turn, 
have condensed upon themselves nebular matter, and 

have thereby become incandescent, if they were not so 



before. The rotational movement in those parts which 
were far removed from the centre of the Milky Way may 
be neglected. At a later period collisions succeeded be- 
tween the single stars which had been gathered up, and 
it is for this reason that gaseous nebulae, as well as new 
stars, are comparatively frequent phenomena in the plane 
of the Milky Way. This view may some day receive 

Fig. 57. Star cluster in Pegasus. Messier 15. Taken at the Yerkes 
Observatory. Scale, 1 mm. =6.4 sec. of arc 

confirmation, when we succeed in proving the existence 
of a central body in the Milky Way, evidence of which 



might possibly be deduced from the curvature of the 
orbits of the sun or of other stars. 

As regards the ring-shaped nebula in the Lyre (Fig. 
50), the most recent measurements made by Newkirk 

point to the result that the 
star visible in its centre is 
distant from us about thirty- 
two light-years. As it ap- 
pears probable that this star 
really forms the central core 
of the nebula, the distance of 
the nebula itself must be 
thirty- two light-years. From 
the diameter of the ring- 
shaped nebula which Newkirk 
estimates at one minute of 
arc, this astronomer has cal- 
culated that the distance of 
the ring from its central bo;ly 
is equal to about three hun- 
dred times the radius of the earth's orbit that is to 
say, the ring is about ten times as far from its sun as 
Neptune is from our sun. There is a faint luminescence 
within this ring. The nebular matter may originally 
have been more concentrated at this spot than in the 
outer portions of the ring itself. But this mass was prob- 
ably condensed on meteors which immigrated from out- 
side, and when these meteors coalesced dark planets were 
produced which move about the central body, and which 
have gathered about them most of the gases. If that 
central body were as heavy as our sun, the matter in 
the ring should revolve about it in five thousand years. 
That rotation would suffice to wipe out the original spiral 
shape, enough of which has yet been left to permit of our 


Fig. 58. Cone-shaped star 
cluster in Gemini. 


distinctly discerning the two wings of the spiral. The 
central body of this ring-shaped nebula gives a continuous 
spectrum of bright lines which is particularly developed 
on the violet side. The star would therefore appear to 
be much younger and much hotter than our sun, and its 
radiation pressure would therefore be much more intense. 
The period of rotation of the nebula may, for this reason, 
have to be estimated at a considerably higher figure. 

The eminent Dutch astronomer Kapteyn has deduced 
from the proper motions of 168 nebulae that their average 
distance from the earth is about seven hundred light-years 
and equal to that of stars of the tenth magnitude. The 
old idea, that the nebulae must be infinitely farther re- 
moved from us than the fainter stars, would therefore ap- 
pear to be erroneous. According to the measurements of 
Professor Bohlin, the nebula in Andromeda may indeed be 
at a distance of not more than forty light-years. 

The "new stars" form a group among the peculiar 
celestial bodies which on account of their variable light 
intensity have been designated as "variable stars," and 
of which a few typical cases should be mentioned, because 
a great scientific interest attaches to these problems. The 
star Eta, in Argus, may be said to illustrate the strange fate 
that a star has to pass through when it has drifted into a 
nebula filled with immigrated celestial bodies. It is one 
of the most peculiar variable stars. The star shines 
through one of the largest nebular clouds in the heavens. 
Whether it stands in any physical connection with its 
surroundings cannot be stated without further examina- 
tion. The star might, for instance, be at a considerable 
distance in front of the nebula, between the latter and 
ourselves. Its frequent change in light intensity sug- 
gests, however, a series of collisions, which do not ap- 
pear unnatural to us when we suppose that the star is 



within a nebula into which many celestial bodies have 

As this star belongs to the southern hemisphere, it was 
not observed before our astronomers commenced to visit 
that hemisphere. In 1677 it was classed as a star of the 
fourth magnitude; ten years later it was of the second 
magnitude; the same in 1751. In 1827 it was of the 
first magnitude, and it was found to be variable that 
is to say, it shone with variable brightness. Herschel ob- 
served that it fluctuated between the first and second 
magnitudes, and that it increased in brightness after 

1837, so that it was by 1838 of magnitude 0.2. After 
that it began to decrease in intensity up to April, 1839, 
when it had the magnitude 1.1. It remained for four 
years approximately at this intensity; then it increased 
rapidly again in 1843, and surpassed all stars except 
Sirius (magnitude 1.7). l Afterwards its intensity slow- 
ly diminished once more, so that it remained just visible 
to the naked eye (sixth magnitude) ; by 1869 it had be- 
come invisible. Since then it has been fluctuating be- 
tween the sixth and seventh magnitudes. 

The last changes in the intensity of this star strongly 
recall the behavior of the new star in Perseus, only that 
the latter has been passing through its phases at a much 
more rapid rate. It appears to be certain, however, that 
Eta, in Argus, was from the very beginning far brighter 
than Nova Persei, and that at least once before the great 
collision in 1843 (after which it was surrounded by ob- 
scuring clouds of increasing opacity) namely, in January, 

1838, it had been exposed to a slight collision of quickly 

1 This figure, -1.7, signifies that the brightness of Sirius is 2.52 2 - 7 = 12 
times greater than that of a star of magnitude 1. Next to Sirius 
comes Canopus, with magnitude 1.0, being 6.3 times brighter than 
a star of magnitude 1. 



vanishing effect. This lesser collision was probably of 
the kind which Mayer imagined for the earth and sun. 
It would give rise to heat development corresponding to 
the heat expenditure of the sun in about a hundred years. 
As it had been observed that the star was variable in 
an irregular manner before that, we ma}^ perhaps, pre- 
sume that it had already undergone another collision. 

According to the observations of Borisiak, a student 
in Kief, the new star in Perseus would have been, on the 
evening of February 21, 1901, of 1.5 magnitude, while 
a few hours previously it had been of magnitude 12, and 
the following evening of magnitude 2.7; afterwards its 
intensity increased up to the following evening, when it 
outshone all the other stars in the northern sky. If this 
statement is not based on erroneous observations, the 
new star must have been in collision with another celestial 
body two days before its great collision, perhaps with a 
small planet in the neighborhood of the sun, with which 
it later collided. That would account for its temporary 

New stars are by no means so rare as one might perhaps 
assume. Almost every year some new star is discovered. 
By far most of these are seen in the neighborhood of the 
Milky Way, where the visible stars are unusually crowded, 
so that a collision which would become visible to us may 
easily occur. 

For similar reasons we find there also most of the 
gaseous nebulae. 

Most of the star clusters are also in the neighborhood 
of the Milky Way. This is in consequence of the facts 
just alluded to. The nebulae which are produced by 
collisions between two suns are soon crossed by migrat- 
ing celestial bodies such as meteorites or comets, which 
there occur in large numbers; by the condensing action 



of these intruders they are then transformed into star 
clusters. In parts of the heavens where stars are rel- 
atively sparse (as at a great distance from the Milky 
Way), most of the nebulae observed exhibit stellar spectra. 
They are nothing but star clusters, so far removed from 
us that the separate stars can no longer be distinguished. 
That single stars and gaseous nebulae are so rarely per- 
ceived in these regions is, no doubt, due to their great 

Among the variable stars we find quite a number which 
display considerable irregularity in their fluctuations of 
brightness, and which remind us of the new stars. To 
this class belongs the just-mentioned star Eta, in Argus. 
Another example (the first one which was recognized as 
"variable") is Mira Ceti, which may be translated, "The 
Wonderful Star in the Constellation of the Whale." This 
mysterious body was discovered by the Frisian priest 
Fabricius, on August 12, 1596, as a star of the second 
magnitude. The priest, an experienced astronomer, had 
not previously noticed this star, and he looked for it in 
vain in October, 1597. In the years 1638 and 1639 the 
variability of the star was recognized, and it was soon 
ascertained to be irregular. The period has a length of 
about eleven months, but it fluctuates irregularly about 
this figure as a mean value. At its greatest intensity 
the star ranks with those of the first or second order. 
Sometimes it is weaker, but it is always of more than the 
fifth magnitude. Ten weeks after a maximum the star 
is no longer visible, and its brightness may diminish to 
magnitude 9.5. In other words, its intensity varies about 
in the ratio of 1 : 1000 (or possibly more). After a mini- 
mum the brightness once more increases, the star be- 
comes visible again that is to say, it attains the sixth 
magnitude and after another six weeks it will once 



more be at its maximum. We have evidently to deal 
with several superposed periods. 

The spectrum of this star is rather peculiar. It be- 
longs to the red stars with a band spectrum which is 
crossed by bright hydrogen lines. The star is receding 
from us with a velocity of not less than 63 km. (39 miles) 
per second. The bright hydrogen lines which correspond 
to the spectrum of the nebula may sometimes be resolved 
into three components, of which the middle 'one corre- 
sponds to a mean velocity of 60 km., and the two others 
have variable receding velocities of 35 and 82 km. that 
is to say, velocities of 25 or 22 km. less or more than the 
mean velocity. Evidently the star is surrounded by three 
nebulae; one is concentrated about its centre; the two 
others lie on a ring the matter of which has been con- 
centrated on two opposite sides. The ring, which recalls 
the ring nebula in the Lyre, seems to move about the star 
with a velocity of 23.5 km. per second. As this revolution 
is accomplished within eleven or, more correctly, within 
twenty-two months, since there must be two maxima 
and two minima during one revolution the total circum- 
ference of the ring will be 23.5 x 86,400 x 6701361 
millions, and the radius of its orbit 217 million km., 
which is 1.45 times greater than the radius of the earth's 
orbit. Now the velocity of the earth in its orbit is 29.5 
km. (18.3 miles) per second. A planet at 1.45 times that 
distance from the sun would have the (1.203 times smaller) 
velocity of 24.5 km. per second, which is approximately 
that of the hypothetical ring of Mira Ceti. We conclude, 
therefore, that the mass of the central sun in Mira Ceti 
will nearly equal the mass of our sun. The calculation 
really suggests that Mira would be about eight per cent, 
smaller; but the difference lies within the range of the 
probable error. 



Chandler has directed attention to a striking regularity 
in these stars. The longer the period of their variation, 
the redder in general their color. This is easily compre- 
hended. The denser the original atmosphere, the more 
widely the gases will have extended outward from the 
star, and the more dust will have been caught or secreted 
by it. We have seen that the limb of the sun has a 
reddish light because of the quantities of dust in the solar 
atmosphere. The effect is chiefly to be ascribed to the 
absorption of the blue rays by the dust; but it may 
partly be explained on the assumption that the solar 
radiations render the dust incandescent, though its tem- 
perature may be lower than that of the photosphere, be- 
cause the dust lies outside the sun, and that it will there- 
fore emit a relatively reddish light. The more dust there 
is in a nebula, the redder will be its luminescence ; and as 
the quantity of dust increases in general with the extension 
of the nebula, that star which is surrounded by wider 
rings of nebulae will in general be more red ; but the greater 
the radius of the ring, the longer also will in general be 
its period. 

The so-called red stars show, in addition to the' bright 
hydrogen lines, banded spectra which indicate the pres- 
ence of chemical compounds. On this account such stars 
were formerly credited with a lower temperature. But 
the same peculiarity is also observed in sun-spots, al- 
though the latter, on account of their position, must have 
a higher temperature than the surrounding photosphere. 
The presence of bands in the spectrum certainly suggests 
high pressure, however. The red stars are evidently sur- 
rounded by a very extensive atmosphere of gases, in the 
inner portions of which the pressure is so high that the 
atoms enter into combination. The spectra of the red 

stars display, on the whole, a striking resemblance to 



those of the sun-spots. The violet portion of the spectrum 
is weakened, because the masses of dust have extinguished 
this light. Owing to the large masses of dust which lie in 
our line of sight, the spectrum lines are :n both cases 
markedly widened and sometimes accompanied by bright 

Another class of stars, distinguished by bright lines, 
comprises those studied by Wolf and Rayet, and named 
after them. These stars are characterized by a hydrogen 
atmosphere of enormous extension, large enough in some 
cases, it has been calculated, to fill up the orbit of Nep- 
tune. These stars are evidently either hotter and more 
strongly radiating than the red stars, or there is not 
so much dust in their neighborhood the dust may pos- 
sibly have been expelled by the strong radiating press- 
ure. They are, therefore, classed with the yellow, and 
not with the red stars. Although there is every reason 
to suppose that their central bodies are at least as hot as 
those of the white stars, the dust is yet able to reduce the 
color to yellow, owing to the vast extensions of their 

The unequal periods in stars like Mira may be ex- 
plained by the supposition that there are several rings 
of dust moving about them, as in the case of the planet 
Saturn, In the case of the inner rings which have a short 
period, there has probably been sufficient time during 
the uncounted number of revolutions to effect a uniform 
distribution of the dust. Hence we do not discern any 
noteworthy nuclei in them, such as we observe in the 
tails of comets; the dust rings only help to impart to 
the star a uniform reddish hue. In the outer rings the 
distribution of dust will, however, not be uniform. One 
of the rings may be responsible for the chief proper 

period. By the co-operation of other less important 
'3 183 


dust rings, the maximum or minimum, we shall easily 
understand, may be displaced, and thus the time interval 
between the maxima and minima be altered. This al- 
teration of the period is so strong for some stars that we 
have not yet succeeded in establishing any simple perio- 
dicity. The best-known star of this type is the bright- 
red star Betelgeuse in the constellation of Orion. The 
brightness of this star fluctuates irregularly between the 
magnitudes 1.0 and 1.4. 

By far the largest number of variable stars belong to 
the type of Mira. Others resemble the variable star 
Beta in the constellation of the Lyre, and thus belong to 
the Lyre type. The variability of the spectra of a great 
many of these stars indicates that they must be moving 
about a dark star as companion, or rather that they both 
move about a common centre of gravity. The change 
in the light intensity is, as a rule, explained by the sup- 
position that the bright star is partially obscured at 
times by its dark companion. Many irregularities, how- 
ever, in their periods and other circumstances prove that 
this explanation is not sufficient. The assumption of 
rings of dust circulating about the star and of larger 
condensation centres affords a better elucidation of the 
variability of these stars. They are grouped with the 
white or yellow stars, in whose surroundings the dust 
does not play so large a part as in that of Mira Ceti. 
The period of their variability is, as a rule, very short, 
moreover generally only a few days (the shortest known, 
only four hours) while the period of the Mira stars 
amounts to at least sixty-five days, and may attain two 
years. There may be still longer periods so far not in- 

Nearly related to the Lyre stars are the Algol stars, 
whose variability can be explained bv the assumption 



that another bright or dark star is moving within 
their vicinity, partially cutting off their light. There 
is no dust in these cases, and the spectrum charac- 
terizes these stars as stars of the first class that is, as 
white stars so far as they have been studied up to the 

We must presume for all the variable stars that the 
line of sight from the observer to the star falls in the 
plane of their dust rings or of their companions. If this 
were not so, they would appear to us like a nebula with 
a central condensation nucleus, or, so far as Algol stars 
are concerned, like the so-called spectroscopic doubles 
whose motion about each other is recognized from the 
displacement of their spectral lines. 

The evolution of stars from the nebulous state has been 
depicted by the famous chief of the Lick Observatory, in 
California, W. W. Campbell, as follows (compare the 
spectra of the stars of the 2d, 3d, and 4th class, Figs. 59 
and 60) : 

440 450 460 470 480 

Fig. 59. Comparison of spectra of stars of classes 2, 3, 4. After 
photographs taken at the Yerkes Observatory. Blue portions 
of spectrum. Wave-lengths in millionths of a millimetre 



520 530 540 550 5f>0 570 580 

Fig. 60. Comparison of spectra of stars of classes 2, 3, 4. After 
photographs taken at the Yerkes Observatory. Green and yellow 
portions of spectrum. Wave-lengths in millionths of a millimetre 

"It is not difficult to select a long list of well-known 
stars which cannot be far removed from nebular condi- 
tions. These are the stars containing both the Hug- 
gins and the Pickering series of bright hydrogen lines, 
the bright lines of helium, and a few others not yet iden- 
tified. Gamma Argus and Zeta Puppis are of this class. 
Another is DM +30.3639, which is actually surrounded 
with a spherical atmosphere of hydrogen some five sec- 
onds of arc in diameter. A little further removed from 
the nebular state are the stars containing both bright 
and dark hydrogen lines caught, so to speak, in the act 
of changing from bright-line to dark-line stars. Gamma 
Cassiopeia, Pleione, and My Centauri are examples. 
Closely related to the foregoing are the helium stars. 
Their absorption lines include the Huggins hydrogen series 
complete, a score or more of the conspicuous helium lines, 



frequently a few of the Pickering series, and usually some 
inconspicuous metallic lines. The white stars in Orion 
and in the Pleiades are typical of this age. 

" The assignment of the foregoing types to an early place 
in stellar life was first made upon the evidence of the 
spectroscope. The photographic discovery of nebulous 
masses in the regions of a large proportion of the bright- 
line and helium stars affords extremely strong confirma- 
tion of their youth. Who that has seen the nebulous 
background of Orion (Fig. 51) or the remnants of neb- 
ulosity in which the individual stars of the Pleiades 
(Fig. 52) are immersed can doubt that the stars in these 
groups are of recent formation? 

"With the lapse of time, stellar heat radiates into 
space, and, so far as the individual star is concerned, is 
lost. On the other hand, the force of gravity on the 
surface strata increases. The inevitable contraction is 
accompanied by increasing average temperature. Changes 
in the spectrum are the necessary consequence. The sec- 
ond hydrogen series vanishes, the ordinary hydrogen ab- 
sorption is intensified, the helium lines become indistinct, 
and calcium and iron absorptions begin to assert them- 
selves. Vega and Sirius are conspicuous examples of this 
period. Increasing age gradually robs the hydrogen lines 
of their importance, the H and K lines broaden, the metal- 
lic lines develop, the bluish-white color fades in the direc- 
tion of the yellow, and, after passing through types 
exemplified by many well-known stars, the solar stage is 
reached. The reversing layer in solar stars represents 
but four or five hydrogen lines of moderate intensity ; the 
calcium lines are commandingly permanent, and some 
twenty thousand metallic lines are visible. The solar 
type seems to be near the summit of stellar life. The 
average temperature of the mass must be nearly a max- 



imum ; for the low density indicates a constitution 
that is still gaseous [compare Chapter VII.]. 

" Passing time brings a lowering of the average tempera- 
ture. The color passes from yellow to red, in consequence 
of lower radiation, temperature, and increasing general 
absorption by the atmosphere. The hydrogen lines be- 
come indistinct, metallic absorption remains permanent, 
and broad absorption bands are introduced. In one 
type (Secchi's Type III.), of which Alpha Herculis is an 
example, these bands are of unknown origin. In another 
class (Secchi's Type IV.), illustrated by the star 19 Pis- 
cium, they have been definitely identified as of carbon 

" There is scarcely room for doubt that these types of 
stars (Type IV.) are approaching the last stages of stellar 
development. Surface temperatures have been lowered 
to the point of permitting more complex chemical com- 
binations than those in the sun. 

" Secchi's Type III. includes the several numbered long- 
period variable stars of the Mira Ceti class, whose spectra 
at maximum brilliancy show several bright lines of 
hydrogen and other chemical elements. 1 It is significant 
that the dull-red stars are all very faint; there are none 
brighter than magnitude 5.5. Their effective radiatory 
power is undoubtedly very low." 

The state of evolution, which succeeds that character- 

1 This circumstance indicates that the red color of these stars, as 
we have already remarked with regard to Mira Ceti, is not to be 
traced back to a low temperature, but rather to the dust surround! ng 
them. The most extraordinary brightness of some stars, like Arc- 
turus and Betelgeuse, which are redder than the sun, and whose 
spectra, according to Hale, resemble those of the sun-spots, pre- 
suppose a very high temperature. The characteristic lines of their 
spectra are produced by the relatively cool vapors of their outer 



ized as the Secchi Type IV., may be elucidated with the 
aid of the examples of Jupiter and the earth, with which 
we are more familiar. These planets would be invisible 
if they were not shining in borrowed light. 

Jupiter has not advanced so far as the earth. The 
specific gravity of Jupiter is somewhat lower than that 
of the sun (1.27 against 1.38), and, apart from the clouds 
in its atmosphere, this planet is probably altogether in 
a gaseous condition, while the earth, with its mean 
density of 5.52, possesses a solid cold crust, enclosing 
its incandescent interior. This state of the earth cor- 
responds to the last stage in the evolution of the stars. 

Of the streams of gaseous matter which are ejected 
when stars collide with one another, the metallic vapors 
are rapidly condensed by cooling; only helium and hy- 
drogen will remain in the gaseous condition and form 
nebular masses about the central body. These nebula? 
yield bright lights. Their luminosity is clue to the nega- 
tive particles which are sent to them by the radiation 
pressure of near stars, and especially by the central bodies 
of the nebula. 

With the new stars which have so far been observed, 
this pressure of radiation soon diminishes, and the nebu- 
lar light likewise decreases in such cases. In other in- 
stances, as with the stars characterized by bright hydro- 
gen and helium lines, the radiation of the central body 
or stars in their vicinity seems to be maintained at full 
force for long periods. 

The nebulous accumulations of helium and hydrogen 
will gradually escape and be condensed in neaf-by stars 
under the formation of "explosive" compounds. The 
tendency to enter into combination seems to be strongest 
in the case of helium; it disappears first from the stellar 

atmosphere. That helium enters into compounds at 



high temperatures seems to follow from the researches 
of Ramsay, Cooke, and Kohlschiitter. 

Hydrogen will afterwards be absorbed, and the light 
of the central body will then show the predominating 
occurrence of the vapors of calcium and of other metals 
in its atmosphere. Simultaneously with these, chemical 
compounds will be noticed, among which the carbon com- 
pounds will play an important part in the outer por- 
tions of the sun-spots, in the stars of the Secchi Type IV., 
as well as in the gaseous envelopes of the cornets. 1 

Finally a crust will form. The star is extinct. 

1 The presence of carbon bands in the spectrum need not be taken 
as a mark of low temperature. Crew and Hale have observed that 
these bands gradually vanished from an arc spectrum as the tem- 
perature was lowered by decreasing the current intensity. 



WE will now proceed to a more intimate consideration 
of the chemical and physical conditions which probably 
characterize the nebula} in distinction from the suns. 
These properties differ in many respects essentially from 
those which we are accustomed to associate with matter 
as investigated by us, which may, from this point of view, 
be styled relatively concentrated. 

The differences must be fundamental. For the motto 
of Clausius, which comprises the sum of our knowledge 
of the nature of heat, cannot apply to nebulse. This 
motto reads: 

" The energy of the universe is constant. The entropy of the 
universe tends to a maximum." 

Everybody understands what is meant by energy. We 
know energy in many forms. The most important are: 
energy of position (a heavy body has larger energy by 
virtue of its having been raised to a certain height above 
the surface of the earth than when it is lying on the 
surface); energy of motion (a discharged rifle-bullet has 
an energy which is proportional to the mass of the bullet 
and to the square of its velocity); energy of heat, which 
is regarded as the energy of the motion of the smallest 
particles of a body; electrical energy, such as can, for 
instance, be stored in an accumulator battery, and which, 



like all other modifications of energy, may be converted 
into energy of heat; and chemical energy, such as is 
displayed by a mixture of eight grammes of oxygen 
with one gramme of hydrogen, which can be trans- 
formed into water under a strong evolution of heat. 
When we say that the energy of a system to which energy 
is not imparted from outside is constant, we merely mean 
that the different forms of energy of the separate parts 
of this system may be transformed into other forms of 
energy, but that the sum total of all the energies must 
always remain unchanged. According to Clausius this 
law is valid throughout the infinite space of the universe. 

By entropy we understand the quantity of heat of a 
body divided by its absolute temperature. If a quantity 
of heat, of Q calories, of a body at a temperature of 100 
(absolute temperature, 373) passes over to another body 
of (absolute temperature, 273), the total entropy of 
the two will have been decreased by yf^, and increased by 
^TS. As the latter quantity is the greater, the entropy 
of the whole will have increased. By itself, we know, 
heat always passes, either by radiation or by conduction, 
from bodies of higher temperature to bodies of lower tem- 
perature. That evidently implies an increase in entropy, 
and it is in agreement with the law of Clausius that 
entropy tends to increase. 

The most simple case of heat equilibrium is that in 
which we place a number of bodies of unequal tempera- 
tures in an enclosure which neither receives heat from 
outside nor communicates heat to the outside. In some 
way or other, usually by conduction or radiation, the 
heat will pass from the warmer to the colder bodies, 
until at last equilibrium ensues and all the bodies have 
the same temperature. According to Clausius, the uni- 
verse tends to that thermal equilibrium. If it be ever 



attained, all sources of motion, and hence of light, will 
have been exhausted. The so-called "heat - death" 
(Warmetod) will have come. . 

If Clausius were right, however, this heat-death, we 
may object, should already have occurred in the infinitely 
long space of time that the universe has been in existence. 
Or we might argue that the world has not yet been in 
existence sufficiently long, but that, anyhow, it had a 
beginning. That would contradict the first part of the 
law of Clausius, that the energy of the universe is con- 
stant; for in that case all the energy would have origi- 
nated in the moment of creation. That is quite incon- 
ceivable, and we must hence look for conditions for which 
the entropy law of Clausius does not hold. 

The famous Scotch physicist Clerk-Maxwell has con- 
ceived of such a case. Imagine a vessel which is divided 
by a partition into two halves, both charged with a gas 
of perfectly uniform temperature. Let the partition be 
provided with a number of small holes which would not 
allow more than one gas molecule to pass at a time. In 
each hole Maxwell places a small, intelligent being (one 
of his "demons"), which directs all the molecules which 
enter into the hole, and which have a greater velocity 
than the mean velocity of all the molecules, to the one 
side, and which sends to the other side all the molecules 
of a smaller velocity than the average. 1 All the undesir- 
able molecules the demon bars by means of a little flap. 
In this way all the molecules of a velocity greater than 
the average may be collected in the one compartment, 
and all the molecules of a lesser velocity in the other com- 

1 The kinetic theory of gases imagines all the molecules of a gas to 
be in constant motion. The internal pressure of the gas depends 
upon the mean velocity of the particles; but some particles will move 
at a greater, and some at a smaller velocity than the average. H.B. 



partment. In other words, heat for heat consists in 
the movements of molecules will pass from the one 
constantly cooling side to the other, which is constantly 
raising its temperature, and which must therefore become 
warmer than the former. 

In this instance heat would therefore pass from a 
colder to a warmer body, and the entropy would di- 

Nature, of course, does not know any such intelligent 
beings. Nevertheless, similar conditions may occur in 
celestial bodies in the gaseous state. When the mole- 
cules of gas in the atmosphere of a celestial body have 
a sufficient velocity which in the case of the earth would 
be 11 km. (7 miles) per second and when they travel 
outward into the most extreme strata, they may pass 
from the range of attraction out into infinite space, after 
the manner of a comet, which, if endowed with sufficient 
velocity when near the sun, must escape from the solar 
system. According to Stoney, it is in this way that the 
moon has lost its original atmosphere. This loss of gas 
is certainly imperceptible in the case of our sun and of 
large planets like the earth. But it may play an impor- 
tant part in the household of the nebulae, where all the 
radiation from the hot celestial bodies is stored up, and 
where, owing to the enormous distances, the restraining 
force of gravity is exceedingly feeble. Thus the nebulae 
will lose their most rapid molecules from their outer 
portions, and they will therefore be cooling in these outer 
strata. This loss of heat is compensated by the radiation 
from the stars. If, now, there were only nebulae of one 
kind in the whole universe, those escaped molecules would 
finally land on some other nebula, heat equilibrium would 
thus be established between the different nebulae, and the 

" heat-death " be realized. But we have already remark- 


ed that the nebulae enclose many immigrated celestial 
bodies, which are able to condense the gases from their 
neighborhood, and which thereby assume a higher tem- 

The lost molecules of gases may also stray into the 
vast atmosphere of these growing stars, and the con- 
densation will then be hastened under a continuous lower- 
ing of the entropy. By such processes the clock-work 
of the universe may be maintained in motion without 
running down. 

About the bodies which have drifted into nebulae, and 
about the remnants of new stars which lie inside the 
nebulae, the gases will thus collect which had formerly 
been scattered through the outer portions of the nebula. 
These gases originate from the explosive compounds 
which had been stored in the interior of the new stars. 
Hydrogen and helium are, most likely, the most impor- 
tant of these; for they are the most difficult to be con- 
densed, and can exist in notable quantities at extremely 
low temperatures, such as must prevail in the outermost 
portions of the nebulae, in which gases of other substances 
would be liquefied. Even if the nebulae had an absolute 
temperature of 50 (-223 C.), the vapor of the most 
volatile of all the metals, mercury, would even in the 
saturated state be present in such a small quantity that 
a single gramme would occupy the space of a cube whose 
side would correspond to about two thousand light-years 
that is to say, to 450 times the distance of the earth 
from the nearest fixed star. One gramme of sodium, like- 
wise a very volatile metal, and of a comparatively high 
importance in the constitution of the fixed stars, would 
fill the side of a cube that would be a thousand million 
times as large. Still more inconceivable numbers re- 
sult for magnesium and iron, which are very frequent 



constituents of fixed stars, and which are less volatile 
than the just-mentioned metals. We thus recognize the 
strongly selective action of the low temperatures upon 
all the substances which are less difficult to condense than 
helium and hydrogen. As we now know that there is 
another substance in the nebulae, which has been desig- 
nated nebulium, and which is characterized by two spec- 
tral lines not found in any terrestrial substance, we must 
conclude that this otherwise unknown element nebulium 
must be almost as difficult to condense as hydrogen and 
helium. Its boiling-point will probably lie below 50 
absolute, like that of those gases. 

That hydrogen and helium, together with nebulium, 
alone seem to occur in the vastly extended nebulae is 
probably to be ascribed to their low boiling-points. We 
need not look for any other explanation. The supposi- 
tion of Lockyer that all the other elements would be trans- 
formed into hydrogen and helium at extreme rarefaction 
is quite unsupported. 

In somewhat lower strata of the nebula, where its 
shape resembles a disk, other not easily condensable 
substances, such as nitrogen, hydro-carbons of simple 
composition, carbon monoxide, further, at deeper levels, 
cyanogen and carbon dioxide, and, near the centre, sodium, 
magnesium, and even iron may occur in the gaseous state. 
These less volatile constituents may exist as dust in the 
outermost strata. This dust would not be revealed to us 
by the spectroscope'. In the strongly developed spiral 
nebulae, however, the extreme layers, which seem to hide 
the central body, appear to be so attenuated that the 
dust floating in them is not able to obscure the 
spectrum of the metallic gases. The spectrum of the 
nebula then resembles a star spectrum, because the 
deepest strata contain incandescent layers of dust clouds, 



whose light is sifted by the surrounding masses of 

It has been observed that the lines of the different 
elements are not uniformly distributed in the nebula?. 
Thus Campbell observed, for instance, when investigating 
a small planetary nebula in the neighborhood of the great 
Orion nebula, that the nebulium had not the same dis- 
tribution as the hydrogen. The nebulium, which was con- 
centrated in the centre of the nebula, probably has a 
higher boiling-point than hydrogen, therefore, and occurs 
in noticeable quantities in the inner, hotter parts of the 
nebula. Systematic investigations of this kind may help 
us to a more perfect knowledge of the temperature rela- 
tions in these peculiar celestial objects. 

Ritter and Lane have made some interesting calcula- 
tions on the equilibrium in a gaseous celestial body of 
so low a density that the law of gases may be applied to 
it. That is only permissive for gases or for mixtures of 
gases whose density does not exceed one-tenth of that 
of water or one-fourteenth of the actual density of the 
sun. The pressure in the central portions of such a mass 
of gas would, of course, be greater than the pressure in 
the outer portions, just as the pressure rises as we pene- 
trate from above downward into our terrestrial atmos- 
phere. If we imagine a mass of the air of our atmosphere 
transferred one thousand metres higher up, its volume will 
increase and its temperature will fall by 9.8 C. (18 F.). 
If there were extremely violent vertical convection cur- 
rents in the air, its . temperature would diminish in this 
manner with increasing altitude; but internal radiation 
tends to equalize these temperature differences. The 
following calculation by Schuster concerning the con- 
ditions of a mass of gas of the size of the sun is based 
on Ritter's investigation. It has been made under the 




hypothesis that the thermal properties of this mass of 
gas are influenced only by the movements in it, and not 
by radiation. The calculation is applied to a star which 
has the same mass as the sun (1.9x10" grammes, or 
324,000 times the mass of the earth), and a radius of 
about ten times that of the sun (10x690,000 km.), whose 
mean density would thus be 1000 times smaller than that 
of the sun, or 0.0014 times the density of water at 4 C. 
In the following table the first column gives the distance 
of a point from the centre of the star as a fraction of 
its radius; the density (second column) is expressed in 
the usual scale, water being the unit ; pressures are stated 
in thousands of atmospheres, temperatures in thousands 
of degrees Centigrade. The temperature will vary pro- 
portionately to the molecular weight of the gas of which 
the star consists; the temperatures, in the fourth column 
of the table, concern a gas of molecular weight 1 that 
is to say, hydrogen gas dissociated into atoms, as it 
will be undoubtedly on the sun and on the star. If the 
star should consist of iron, we should have to multiply 
these latter numbers by 56, the molecular weight of iron ; 
the corresponding figures will be found in the fifth column. 

Temperature in 

Distance from 


Pressure in 10 3 


























































Schuster's calculation was really made for the sun 
that is to say, for a celestial body whose diameter is ten 



times smaller, and whose specific gravity is therefore a 
thousand times greater than the above-assumed values. 
According to the laws of gravitation and of gases, the 
pressure must there be 10,000 times greater, and the 
temperature ten times higher, than those in our table. 
The density of the interior portions would, however, be- 
come far too large to admit of the application of the 
gas laws. I have therefore modified the calculations so 
as to render them applicable to a celestial body of ten 
times the radius of the sun or of 1080 times the radius 
of the earth ; the radius would then represent one-twenty- 
second of the distance from the centre of the sun to the 
earth's orbit, and the respective celestial body would have 
very small dimensions indeed if compared to a nebula. 

The extraordinarily high pressure in the interior por- 
tions of the celestial body is striking; this is due to the 
great mass and to the small distances. In the centre 
of the sun the pressure would amount to 8520 million 
atmospheres, since the pressure increases inversely as the 
fourth power of the radius. The pressure near the centre 
of the sun is, indeed, almost of that order. If the sun 
were to expand to a spherical planetary nebula of a 
thousand times its actual linear dimensions (when it 
would almost fill the orbit of Jupiter), the specific gravity 
at its centre would be diminished to one-millionth of the 
above-mentioned value that is to say, matter in this 
nebula would not, even at the point of greatest concen- 
tration, be any denser than in the highly rarefied vacuum 
tubes which we can prepare at ordinary temperatures. 
The pressure would likewise be greatly diminished 
namely, to about six millimetres only, near the centre of 
the gaseous mass. The temperature, however, would be 
rather high near the centre namely, 24,600 C., if the 
nebula should consist of atomatic hydrogen, and fifty-six 
14 199 


times as high again if consisting of iron gas. Such a 
nebula would restrain gases with 1.63 times the force 
which the earth exerts. Molecules of gases moving out- 
ward with a velocity of about 18 km. (11 miles) per sec- 
ond would forever depart from this atmosphere. 

The estimation of the temperature in such masses of 
gases is certainly somewhat unreliable. We have to pre- 
sume that neither radiation nor conduction exert any 
considerable influence. That might be permitted for 
conduction; but we are hardly justified in neglecting 
radiation. The temperatures within the interior of the 
nebula will, therefore, be lower than our calculated values. 
It is, however, difficult to make any definite allowance 
for this factor. 

If the mass of the celestial body should not be as 
presumed for instance, twice as large we should only 
have to alter the pressure and the density of each layer 
in the same proportion, and thus to double the above 
values. The temperature would remain unchanged. 
We are hence in a position to picture to ourselves the state 
of a nebula of whatever dimensions and mass. 

Lane has proved, what the above calculations also in- 
dicate, that the temperature of such nebula will rise 
when it contracts in consequence of its losing heat. If 
heat were introduced from outside, the nebula would ex- 
pand under cooling. A nebula of this kind presumably 
loses heat and gradually raises its own temperature until 
it has changed into a star, which will at first have an at- 
mosphere of helium and of hydrogen like that of the 
youngest stars (with white light). By-and-by, under a 
further rise of temperature, the extremely energetic 
chemical compounds will be formed which characterize the 
interior of the sun, because helium and. hydrogen which 

were liberated when the nebula was re-formed and which 



dashed out into space will diffuse back into the interior 
of the star, where they will be bound under the formation 
of the compounds mentioned. The atmosphere of hy- 
drogen and of helium will disappear (helium first), the 
star will contract more and more, and the pressure and 
the convection currents in the gases will become enor- 
mous. There will be a strong formation of clouds in the 
atmosphere of the star, which will gradually become en- 
dowed with the properties which characterize our sun. 
The sun behaves very differently from the gaseous nebulae 
for which the calculations of Lane, Ritter, and Schuster 
hold. For when the contraction of a gas shall have pro- 
ceeded to a certain limit, the pressure will increase in 
the ratio 1 : 16, while the volume will decrease in the ratio 
8:1, provided there be no change in the temperature. 
When the gas has reached this point and is still further 
compressed, the temperature will remain in steady equi- 
librium. At still higher pressures, however, the tempera- 
ture must fall if equilibrium is to be maintained. Accord- 
ing to Amagat, this will occur at 17 C. (290 absolute) 
in gases like hydrogen and nitrogen, which at this tem- 
perature are far above their critical points, and at a press- 
ure of 300 or 250 atmospheres. When the temperature 
is twice as high on the absolute scale, or at 307 C., 
twice the pressure will be required. 

We can now calculate when our nebula will pass 
through this critical stage, to which a lowering of the 
temperature must succeed. Accepting the above figures, 
we find that half the mass of the nebula will fill a sphere 
of a radius 0.53 of that of the nebula. If the mass were 
everywhere of the same density, half of it would fill a 
sphere of 0.84 of this radius. When will the interior 
mass cross the boundary of the above stage, while the 
exterior portions still remain below this stage? That 



will be at about the time when the nebula in its totality 
will pass through its maximum temperature. We will 
now base our calculations on the temperatures which 
apply to iron in the gaseous state; for in the interior of 
the nebula the mean molecular weight will at least be 
56 (that of iron). We shall find that the pressure at 
the distance 0.53 will be about 177,000 atmospheres, and 
the temperature approximately 71 million degrees i.e., 
245,000 times higher than the absolute temperature in the 
experiments of Amagat. The specified stage will then 
be reached when the pressure will be 245,000 times as 
large as 250 atmospheres viz., 61 million atmospheres. 
As, now, the pressure is only 177,000 atmospheres, our 
nebula will yet be far removed from that stage at which 
cooling will set in. We can easily calculate that this 
will take place when the nebula has contracted to a 
volume about three times that of our sun. The assertion 
which is so often made that the sun might possibly attain 
higher temperatures in the future is unwarranted. This 
celestial body has long since passed through the culminat- 
ing-point of its thermal evolution, and is now cooling. 
As the temperatures which Schuster deduced were no 
doubt much too high, the cooling must, indeed, have 
set in already in an earlier stage. But stars like Sirius, 
whose density is probably not more than one per cent, 
of the solar density, are probably still in a rising-tempera- 
ture stage. Their condition approximates that of the 
mass of gas of our example. 

The planetary nebulae are vastly more voluminous. 
The immense space which these celestial bodies may oc- 
cupy will be understood from the fact that the largest 
among them, No. 5 in Herschel's catalogue, situated near 
the star B in the Great Bear, has a diameter of 2.67 
seconds of arc. If it were as near to us as our nearest 



star neighbor, its diameter would yet be more than three 
times that of the orbit of Neptune; doubtless it is many 
hundreds of times larger. This consideration furnishes 
us with an idea of the infinite attenuation in such struct- 
ures. In their very densest portions the density cannot 
be more than one-billionth of the density of the air. 
In the outer portions of such nebulae the temperature 
niust also be exceedingly low; else the particles of the 
nebula could not be kept together, and only hydrogen 
and helium can occur in them in the gaseous state. 

Yet we may regard the density and temperature of such 
celestial bodies as gigantic by comparison with those of 
the gases in the spirals of the nebulae. There never is 
equilibrium in these spirals, and it is only because the 
forces in action are so extraordinarily small that these 
structures can retain their shapes for long periods with- 
out noticeable changes. It is, probably, chiefly in those 
parts in which the cosmical dust is stopped in its motion 
that meteorites and comets are produced. The dust 
particles wander into the more central portions of the 
nebulae, into which they penetrate deeply, owing to their 
relatively large mass, to form the nuclei for the growth 
of planets and moons. By their collisions with the masses 
of gases which they encounter, they gradually assume a 
circular movement about the axis of rotation of the 
nebula. In this rotation they condense portions of the 
gases on their surface, and hence acquire a high tem- 
perature which they soon lose again, however, owing to 
the comparatively rapid radiation. 

So far as we know, spiral nebulae are characterized by 
continuous spectra. The splendor of the stars within 
them completely outshines the feeble luminosity of the 
nebula. The stars in them are condensation products 

and undoubtedly in an early stage of their existence; 



they may therefore be likened to the white stars, like 
the new star in Perseus and the central star in the 
ring nebula of the Lyre. Nevertheless, it has been as- 
certained that the spectrum of the Andromeda nebula has 
about the same length as that of the yellow stars. That 
may be due to the fact that the light of the stars in this 
nebula, which we only seem to see from the side, is partly 
extinguished by dust particles in its outer portion, as was 
the case with the light of the new star in Perseus during 
the period of its variability. 

Our considerations lead to the conclusion that there 
is rotating about the central body of the nebula an 
immense mass of gas, and that outside this mass there 
are other centres of condensation moving about the cen- 
tral body together with the masses of gas concentrated 
about them. Owing to the friction between the immi- 
grated masses and the original mass of gas which cir- 
culated in the equatorial plane of the central body, all 
these masses will keep near the equatorial plane, which 
will therefore deviate little from the ecliptic. We thus 
obtain a proper planetary system, in which the planets 
are surrounded by colossal spheres of gas like the stars 
in the Pleiades (Fig. 52). If, now, the planets have very 
small mass by comparison with the central body as in 
our solar system they will be cooled at an infinitely 
faster rate than the sun. The gaseous masses will soon 
shrink, and the periods of rotation will be shortened; but 
for those planets, at least, which are situated near the 
centre, these periods will originally differ little from the 
rotation of the central body. The dimensions of the 
central body will always be very large, and the planets 
circulating about it will produce very strong tidal effects 
in its mass. Its period of rotation will be shortened, 

while the orbital rotation of the planets will tend to be- 



come lengthened. Thus the equilibrium is disturbed; it 
is re-established again, because the planet is, so to say, 
lifted away from the sun, as G. H. Darwin has so in- 
geniously shown with regard to the moon and the earth. 
Similar relations will prevail in the neighborhood of 
those planets which will thus become provided with moons. 
Hence we understand the peculiar fact that all the planets 
move almost in the same plane, the so-called ecliptic, 
and in approximately circular orbits; that they all move 
in the same direction, and that they have the same 
direction of rotation in common with their moons and 
with the central body, the sun. It is only the outermost 
planets, like Uranus and Neptune, in whose cases the tidal 
effects were not of much consequence, that form excep- 
tions to this rule. 

In explanation of these phenomena various philosophers 
and astronomers have advanced a theory which is known 
as the Kant-Laplace theory, after its most eminent ad- 
vocates. Suggestions pointing in the same direction we 
find in Swedenborg (1734). Swedenborg assumed that 
our planetary system had been evolved under the forma- 
tion of vortices from a kind of "chaos solare," which had 
acquired a more and more energetic circulating motion 
about the sun under the influence of internal forces, 
possibly akin to magnetic forces. Finally a ring had been 
thrown off from the equator, and had separated into 
fragments, out of which the planets had been formed. 

Buffon introduced gravitation as the conservational 
principle. In an ingenious essay, "Formation des Pla- 
netes" (1745), he suggests that the planets may have 
been formed from a "stream" of matter which was 
ejected by the sun when a comet rushed into it. 

Kant started from an original chaos of stationary dust, 

which under the influence of gravitation arranged itself 



as a central body, with rings of dust turning around it; 
the rings, later on, formed themselves into planets. The 
laws of mechanics teach, however, that no rotation can 
be set up in a central body, which is originally station- 
ary, by the influence of a central force like gravitation. 
Laplace, therefore, assumed with Swedenborg that the 
primeval nebula from which our solar system was evolved 
had been rotating about the central axis. According to 
Laplace, rings like those of Saturn would split off, as 
such a system contracted, and planets and their moons 
and rings would afterwards be formed out of those rings. 
It is generally believed at present, however, that only 
meteorites and small planets, but not the larger planets, 
could have originated in this way. We have, indeed, 
such rings of dust rotating about Saturn, the innermost 
more rapidly, the outer rings more slowly, just as they 
would if they were crowds of little moons. 

Many further objections have later been raised against 
the hypothesis of Laplace, first by Babinet, later especially 
by Moulton and Chamberlin. In its original shape this 
hypothesis would certainly not appear to be tenable. 
I have therefore replaced it by the evolution thesis out- 
lined above. It is rather striking that the moons of the 
outermost planets, Neptune and Uranus, do not move 
in the plane of the ecliptic, and that their moons further 
describe a "retrograde" movement that is to say, they 
move in the direction opposite to that conforming to the 
theory of Laplace. The same seems to hold for the moon 
of Saturn, which was discovered in 1898 by Pickering. All 
these facts were, of course, unknown to Laplace in 1776; 
and if he had known them he would scarcely have ad- 
vanced his thesis in the garb in which he offered it. The 
explanation of these facts does not cause any difficulty. 

We may assume that the matter in the outer portions of 



the primeval nebula was so strongly attenuated that the 
immigrating planet did not attain a sufficient volume to 
have the large common rotation in the equatorial plane 
of the sun impressed upon it by the tidal effects. Charged 
only with the small mass of matter which they met on 
their road, the planet and its moon, on the contrary, re- 
mained victorious in the limited districts in which they 
were rotating. Only the slow orbital movement about 
the central body was influenced, and that adapted itself 
to the common direction and the circular orbit. It is 
not inconceivable that there may be, farther out in space, 
planets of our solar system, unknown to us, moving in 
irregular paths like the comets. The comets, Laplace 
assumed, probably immigrated at a later period into our 
solar system when the condensation had already ad- 
vanced so far that the chief mass of the nebular matter 
had disappeared from interplanetary space. 

Chamberlin and Moulton have attempted to show that 
the difficulties of the hypothesis of Laplace may be 
obviated by the assumption that the solar system has 
evolved from a spiral nebula, into which strange bodies 
intruded which condensed the nebular mass of their sur- 
roundings upon themselves. We have pointed out ex- 
amples of how the nebula seems to vanish in the vicinity 
of the stars, which would correspond to growing planets, 
located in nebulae. 

In concluding this consideration, we may draw a com- 
parison between the views which were still entertained 
a short time ago and the views and prospects which the 
discoveries of moderji days open to our eyes. 

Up to the beginning of this century the gravitation of 
Newton seemed to rule supreme over the motions and 
over the development of the material universe. By virtue 
of this gravitation the celestial bodies should tend to 



draw together, to unite in ever-growing masses. In the 
infinite space of past time the evolution should have 
proceeded so far that some large suns, bright or extinct, 
could alone persist. All life would be impossible under 
such conditions. 

And yet we discern in the neighborhood of the sun 
quite a number of dark bodies, our planets, and we may 
surmise that similar dark companions or satellites exist 
in the vicinity of other suns and stars; for we could 
not understand the peculiar to-and-fro motions of those 
stars on any other view. We further observe that quite 
a number of small celestial bodies rush through space in 
the shapes of meteorites or shooting-stars which must 
have come to us from the most remote portions of the 

The explanation of these apparent deviations from 
what we may regard as a necessary consequence of the 
exclusive action of gravity will be found under two 
heads in the action of the mechanical radiation press- 
ure of light, and in the collisions between celestial 
bodies. The latter produce enormous vortices of gases 
about nebular structures in the gaseous condition; the 
radiation pressure carries cosmical dust into the vortices, 
and the dust collects into meteorites and comets and 
forms, together with the condensation products of the 
gaseous envelope, the planets and the moons accompany- 
ing them. 

The scattering influence of the radiation pressure there- 
fore balances the tendency of gravitation to concentrate 
matter. The vortices of gases in the nebula only servo 
to fix the position of the dust, which is ejected from the 
suns through the action of the radiation pressure. 

The masses of gas within the nebulae form the most im- 
portant centres of concentration of the dust which is eject- 



ed from the sun and stars. If the world were limited, as 
people used to fancy that is to say, if the stars were 
crowded together in a huge heap, and only infinite, 
empty space outside of this heap, the dust particles ejected 
from the suns during past ages by the action of the radiat- 
ing pressure would have been lost in infinite space, just 
as we imagined that the radiated energy of the sun was 

If that were so, the development of the universe would 
long since have come to an end, to an annihilation of all 
matter and of all energy. Herbert Spencer, among 
others, has explained how thoroughly unsatisfactory this 
view is. There must be cycles in the evolution of the 
universe, he has emphasized. That is manifestly in- 
dispensable if the system is to last. In the more rare- 
fied, gaseous, cold portions of the nebulse we find that 
part of the machinery of the universe which checks the 
waste of matter and, still more, the waste of force from 
the suns. The immigrating dust particles have absorbed 
the radiation of the sun and impart their heat to the 
separate particles of the gases with which they collide. 
The total mass of gas expands, owing to this absorption 
of heat, and cools in consequence. The most energetic 
molecules travel away, and are replaced by new particles 
coming from the inner portions of the nebulse, which are 
in their turn cooled by expansion. Thus every ray 
emitted by a sun is absorbed, and its energy is transferred, 
through the gaseous particles of the nebulse, to suns that 
are being formed and which are in the neighborhood of 
the nebula or in its interior portions. The heat is hence 
concentrated about centres of attraction that have 
drifted into the nebula or about the remnants of the 
celestial bodies which once collided there. Thanks to the 
low temperature of the nebula, the matter can again ac- 



cumulate, while the radiation pressure, as Poynting has 
shown, will suffice to keep bodies apart if their tempera- 
ture is 15 C., their diameter 3.4 cm., and their specific 
gravity as large as that of the earth, 5.5. At the distance 
of the orbit of Neptune, where the temperature is about 
50 absolute and approximates, therefore, that of a 
nebula, this limit of size is reduced to nearly one milli- 
metre. It has already been suggested (compare page 
153) that capillary forces, which would prevail under the 
co-operation of the gases condensed upon the dust grains, 
rather than gravity, play a chief part in the accumu- 
lation and coalescence of the small particles. In the 
same manner as matter is concentrated about centres of 
attraction energy may be accumulated there in contra- 
diction to the law of the constant increase of entropy. 

During this conservation al activity the layers of gas 
are rapidly rarefied, to be replaced by new masses 
from the inner parts of the nebula, until this centre is 
depleted, and the nebula has been converted into a 
star cluster or a planetary system which circulates about 
one or several suns. When the suns collide once more 
new nebulae are created. 

The explosive substances, consisting probably of hy- 
drogen and helium (and possibly of nebulium), in com- 
bination with carbon and metals, play a chief part in 
the evolution from the nebular to the stellar state, and 
in the formation of new nebulae after collisions between 
two dark or bright celestial bodies. The chief laws of 
thermodynamics lead to the assumption that these ex- 
plosive substances are formed during the evolution 
of the suns and are destroyed during their collisions. 
The enormous stores of energy concentrated in these 
bodies perform, in a certain sense, the duty of powerfully 
acting fly - wheels interposed in the machinery of the 



universe in order to regulate its movements and to make 
certain that the cyclic transition from the nebular to 
the star stage, and vice versa, will occur in a regular 
rhythm during the immeasurable epochs which we must 
concede for the evolution of the universe. 

By virtue of this compensating co-operation of gravity 
and of the radiation pressure of light, as well as of tem- 
perature equalization and heat concentration, the evolu- 
tion of the world can continue in an eternal cycle, in 
which there is neither beginning nor end, and in which 
life may exist and continue forever and undiminished. 



WE have just recognized the probability of the assump- 
tion that solar systems have been evolved from nebulae, 
and that nebulae are produced by the collision of suns. 
We likewise consider it probable that there circulate 
about the newly formed suns smaller celestial bodies 
which cool more rapidly than the central sun. When 
these satellites have provided themselves with a solid 
crust, which will partly be covered by water, they may, 
under favorable conditions, harbor organic life, as the 
earth and probably also Venus and Mars do. The satel- 
lites would thereby gain a greater interest for us than 
if we had to imagine them as consisting entirely of life- 
less matter. 

The question naturally arises whether we may believe 
that life can really originate on a celestial body as soon as 
circumstances are favorable for its evolution and prop- 
agation. This question will occupy us in this last 

Men have been pondering over these problems since the 
remotest ages. All living beings, past ages recognized, 
must have been generated and they had to die after a 
certain shorter or longer life. Somewhat later, and yet 
still in a very early epoch, experience must have taught 
men that organisms of one kind can only generate other 

organisms of the same kind; that the species are in- 



variable, as we now express it. The idea was that all 
species originally came from the hands of the Creator 
endowed with their present qualities. This view may 
still be said to represent the general or "orthodox" doc- 

This view has also been called the Linnsean thesis, be- 
cause Linne, in the fifth edition of his Genera Plantarum, 
adheres to it strictly: "Species tot sunt, quot diversas 
formas ab initio produxit Infinitum Ens, quae deinde 
formae secundum generationis inditas leges produxere 
plures, at sibi semper similes, ut species nunc nobis non 
sint plures quam fuerunt ab initio." Which we may 
render: "There are as many different kind of species as 
the Infinite Being has created different forms in the 
beginning. These forms have later engendered other 
beings according to the laws of inheritance, always re- 
sembling them, so that we have at the present time not 
any more species than there were from the beginning." 
Time was ripe, however, even then for a less rigid con- 
ception of nature, more in accordance with our present 
views. The first foundations of the theory of evo- 
lution in the biological sciences were laid by Lamarck 
(in 1794), Treviranus (in 1809), Goethe and Oken (in 1820). 
But a reaction set in. Cuvier and his authority forced 
public opinion back to the ancient stand-point. In his 
view the now extinct species of past geological epochs 
had been destroyed by natural revolutions, and new 
species had again been generated by a new act of the 

Within the last few decades, however, the general be- 
lief has rapidly been revolutionized, and the theory of 
evolution, especially since the immortal Charles Darwin 
came forth with his epoch-making researches, now meets 
with universal acceptance. 



According to this theory the species adapt themselves 
in the course of time to their surroundings, and the 
changes may become so great that a new species may be 
considered to have originated from an old species. The 
researches of De Vries have, within quite recent times, 
further accentuated this view, so that we now concede 
cases to be extant where new species spring forth from 
old ones under our very eyes. This thesis has become 
known as the theory of mutation. 

At the present time we accordingly imagine that living 
organisms, such as we see around us, have all descended 
from older organisms, rather unlike them, of which we 
still find traces and remnants in the geological strata 
which have been deposited during past ages. From this 
stand-point all living organisms might possibly have 
originated from one single, most simple organism. How 
that was generated still remains to be explained. 

The common view, to which the ancients inclined, 
is that the lower organisms need not necessarily have 
originated from seeds. It was noticed that some low- 
type organisms, larvae, etc., took rise in putrid meat; 
Vergil describes this in his Georgicas. It was not 
until the seventeenth century that this belief was dis- 
proved by many experiments, among others by those 
of Swammerdam and Leuwenhoek. The thesis of the 
so-called "Generatio spontanea" once more blossomed 
into new life upon the discovery of the so-called in- 
fusoria, the small animal organisms which seem to 
arise spontaneously in infusions and concoctions. Spal- 
lanzani, however, demonstrated in 1777 that when the 
infusions, and the vessel containing them, as well as the 
air above them, were heated to a sufficiently high tem- 
perature to kill all the germs present, the infusions would 
remain sterile, and no living organisms could develop 



in them. To this fact we owe our ordinary methods of 
making preserves. It is true that objections were raised 
against this demonstration. The air, it was objected, is 
so changed by heating that subsequent development of 
minute organisms is rendered impossible. But this last 
objection was refuted by the chemists Chevreul and 
Pasteur, as well as by the physicist Tyndall in the sixties 
and seventies of the past century. These scientists dem- 
onstrated that no organisms are produced in air which 
is freed from the smallest germs by some other means than 
heating i.e., by nitration through cotton-wool. The 
researches of Pasteur, in particular, and the methods of 
sterilization which are based upon them and which are 
applied every day in bacteriological laboratories, have 
more and more forced the conviction upon us that a germ 
is indispensable for the origination of life. 

And yet eminent scientists take up the pen again and 
again in order to demonstrate the possibility of the 
"Generatio spontanea." In this they do not rely upon 
the safe methods of natural science, but they proceed on 
philosophical lines of argument. Life, they suggest, 
must once have had a beginning, and we are hence forced 
to believe that spontaneous generation, even if not realiz- 
able under actual conditions, must have once occurred. 
Considerable interest was excited when the great Eng- 
lish physiologist Huxley believed he had discovered in 
the mud brought up from the very bottom of the sea 
an albuminoid substance which he called "Bathybius 
Haeckelii," in honor of the zealous German Darwinist 
Haeckel. In this bathybius (deep-sea organism) one 
fancied for a time that the primordial ooze, which had 
originated from inorganic matter and from which all 
organisms might have been evolved, and of which Oken 

had been dreaming, had been discovered. But the more 
is 215 


exact researches of the chemist Buchanan demonstrated 
that the albuminoid substance in this primordial ooze 
consisted of flocks of gypsum precipitated by alcohol. 

People then had recourse to the most fantastic specula- 
tions. Life, it was argued, might possibly have had its 
origin in the incandescent mass of the interior of the 
earth. ,At high temperatures organic compounds of 
cyanogen and its derivatives might be formed which 
would be the carriers of life (Pfliiger). There is, however, 
little need of our entering into any of these speculations 
until they have been provided with an experimental 

Almost every year the statement is repeated in 
biological literature that we have at last succeeded in 
producing life from dead matter. Among the most recent 
assertions of this kind, the discovery claimed by Butler- 
Burke has provoked much comment. He asserted that he 
had succeeded, with the aid of the marvellous substance 
radium, in instilling life into lifeless matter namely, a 
solution of gelatine. Criticism has, however, relegated 
this statement, like all similar ones, to the realm of fairy 

We fully share the opinion which the great natural 
philosopher Lord Kelvin has expressed in the following 
words: "A very ancient speculation, still clung to by 
many naturalists (so much so that I have a choice of 
modern terms to quote in expressing it), supposes that, 
under meterological conditions very different from the 
present, dead matter may have run together or crystallized 
or fermented into ' germs of life,' or 'organic cells/ or 
'protoplasm.' But science brings a vast mass of induc- 
tive evidence against this hypothesis of spontaneous gen- 
eration. Dead matter cannot become living without 
coming under the influence of matter previously alive. 



This seems to me as sure a teaching of science as the law 
of gravitation." 

Although the latter verdict may be a little dogmatic, 
it yet demonstrates how strongly many scientists feel 
the necessity of finding another way of solving the prob- 
lem. The so-called theory of panspermia really shows 
a way. According to this theory life-giving seeds are 
drifting about in space. They encounter the plane ts7 
and fill their surfaces with life as soon as the necessary 
conditions for the existence of organic beings are estab- 

This view was probably foreshadowed long ago. Defi- 
nite suggestions in this direction we find in the writings 
of the Frenchman Sales-Guy on de Montlivault (1821), 
who assumed that seeds from the moon had awakened the 
first life on the surface of the earth. The German physi- 
cian H. E. Richter attempted to supplement the doctrine 
of Darwin by combining the conception of panspermia 
with it. Flammarion's book on the plurality of inhabited 
worlds. suggested to Richter the idea that seeds had come 
from some other inhabited world to our earth. He em- 
phasizes the fact that carbon has been found in meteorites 
which move in orbits similar to those of the comets which 
wander about in space ; and in this carbon he sees the rests 
of organic life. There is no proof at all for this latter 
opinion. The carbon found in meteorites has never ex- 
hibited any trace of organic structure, and we may well 
imagine the carbon e.g., that which appears to occur in 
the sun to be of inorganic origin. Still more fantastic is 
his idea that organisms floating high in our atmosphere 
are caught by the attraction of meteorites flying past our 
planet, and are in this way carried out into universal space 
and deposited upon other celestial bodies. As the surface 

of meteorites becomes incandescent in their flight through 



the atmosphere, any germs which they might possibly 
have caught would be destroyed ; and if, in spite of that, 
a meteorite should become the conveyor of live germs, 
those germs would be burned in the atmosphere of the 
planet on which they descended. 

In one point, however, we must agree with Richter. 
There is logic in his statement that " The infinite space is 
filled with, or (more correctly) contains, growing, mature, 
and dying celestial bodies. By mature worlds we under- 
stand those which are capable of sustaining organic life. 
We regard the existence of organic life in the universe 
as eternal. Life has always been there; it has always 
propagated itself in the shape of living organisms, from 
cells and from individuals composed of cells." Man used 
to speculate on the origin of matter, but gave that up 
when experience taught him that matter is indestructible 
and can only be transformed. For similar reasons we 
never inquire into the origin of the energy of motion. 
And we may become accustomed to the idea that life is 
eternal, and hence that it is useless to inquire into its 

The ideas of Richter were taken up again in a popular 
lecture delivered in 1872 by the famous botanist Ferdinand 
Cohn. The best-known expression of opinion on the sub- 
ject, however, is that of Sir William Thomson (later Lord 
Kelvin) in his presidential address to the British Associa- 
tion at Edinburgh in 1871: 

"When two great masses come into collision in space, 
it is certain that a large part of each is melted; but it 
seems also quite certain that in many cases a large quan- 
tity of debris must be shot forth in all directions, much of 
which may have experienced no greater violence than 
individual pieces of rock experience in a landslip or in 
blasting by gunpowder. Should the time when this 



earth comes into collision with another body, comparable 
in dimensions to itself, be when it is still clothed as at 
present with vegetation, many great and small fragments 
carrying seed and living plants and animals would un- 
doubtedly be scattered through space. Hence, and be- 
cause we all confidently believe that there are at present, 
and have been from time immemorial, many worlds of 
life besides our own, we must regard it as probable in 
the highest degree that there are countless seed-bearing 
meteoric stones moving about through space. If at the 
present instant no life existed upon this earth, one such 
stone falling upon it might, by what we blindly call 
natural causes, lead to its becoming covered with vege- 
tation. I am fully conscious of the many objections 
which may be urged against this hypothesis. I will 
not tax your patience further by discussing any of them 
on the present occasion. All I maintain is that I believe 
them to be all answerable." 

Unfortunately we cannot share Lord Kelvin's optimism 
regarding this point. It is, in the first instance, ques- 
tionable whether living beings would be able to survive 
the violent impact of the collision of two worlds. We 
know, further, that the meteorite in its fall towards the 
earth becomes incandescent all over its surface, and any 
seeds on it would therefore be deprived of their germi- 
nating power. Meteorites, moreover, show quite a differ- 
ent composition from that of the fragments from the sur- 
face of the earth or a similar planet. Plants develop 
almost exclusively in loose soil, and a lump of earth fall- 
ing through our atmosphere would, no doubt, be disin- 
tegrated into a shower of small particles by the resistance 
of the atmosphere. Each of these particles would by 
itself flash up like a shooting-star, and could not reach 
the earth in any other shape than that of burned dust. 



Another difficulty is that such collisions, which, as we 
presume, are responsible for the flashing-up of so-called 
new stars, are rather rare phenomena, so that little likeli- 
hood remains of small seeds being transported to our earth 
in this manner. 

The question has, however, entered into a far more 
favorable stage since the effects of radiation have become 

Bodies which, according to the deductions of Schwarz- 
schild, would undergo the strongest influence of solar ra- 
diation must have a diameter of 0.00016 mm., supposing 
them to be spherical. The first question is, therefore: 
are there any living seeds of such extraordinary minute- 
ness? The repty of the botanist is that the so-called 
permanent spores of many bacteria have a size of 0.0003 
or 0.0002 mm., and there are, no doubt, much smaller 
germs which our microscopes fail to disclose. Thus, 
yellow-fever in man, rabies in dogs, the foot-and-mouth 
disease in cattle, and the so-called mosaic disease com- 
mon to the tobacco plant in Netherlandish India, and 
also observed in other countries are, no doubt, para- 
sitical diseases ; but the respective parasites have not yet 
been discovered, presumably because they are too minute 
to be visible under the microscope. 1 

It is, therefore, very probable that there are organisms 
so small that the radiation pressure of a sun would push 
them out into space, where they might give rise to life 
on planets, provided they met with favorable conditions 
for their development. 

We will, in the first instance, make a rough calculation 

1 Meanwhile a large number of organisms which are invisible under 
the ordinary microscope have been rendered visible by the aid of the 
ultra-microscope, among others the presumable microbe of the foot- 
and-mouth disease. 



of what would happen if such an organism were detached 
from the earth and pushed out into space by the radiation 
pressure of our sun. The organism would, first of all, 
have to cross the orbit of Mars; then the orbits of the 
smaller and of the outer planets; and, having passed the 
last station of our solar system, the orbit of Neptune, 
it would drift farther into infinite space towards other 
solar systems. It is not so difficult to estimate the time 
which the smallest particles would require for this journey. 
Let their specific gravity be that of water, which will 
very fairly correspond to the facts. The organisms 
would cross the orbit of Mars after twenty days, the Jupiter 
orbit after eighty days, and the orbit of Neptune after 
fourteen months. Our nearest solar system, Alpha Cen- 
tauri, would be reached in nine thousand years. These 
calculations have been made under the supposition that 
the radiation pressure is four times as strong as gravita- 
tion, which would be nearly correct according to the 
figures of Schwarzschild. 1 

These time intervals required for the organisms to reach 
the different planets of our solar system are not too long 
for the germs in question to preserve their germinating 
power. The estimate is more unfavorable in the case 
of their transference from one planetary system to an- 
other, which will require thousands of years. But we 
shall see further on that the very low temperature of 
those parts of space (about 220 C.) would suspend the 
extinction of the germinating power, as it arrests all chem- 
ical reactions. 

As regards the period during which the germinating 

1 The radiation pressure has here been assumed to be somewhat 
greater than on page 103, because the spores are here regarded as 
opaque, while the drops of hydrocarbons have been regarded as 
partially translucid to luminous rays. 



power can be preserved at ordinary temperature, we 
have been told that the so-called " mummy wheat " which 
had been found in ancient Egyptian tombs was still capa- 
ble of germination. ' Critics, however, have established 
that the respective statements of the Arabs concerning 
the sources of that wheat are very doubtful. The French 
scientist Baudoin asserts that bacteria capable of germina- 
tion were found in a Roman tomb which had certainly 
remained untouched for eighteen hundred years; but 
this statement is to be received with caution. It is cer- 
tain, however, that both seeds of some higher plants and 
spores of certain bacteria e.g., anthrax do maintain 
their germinating power for several years (say, twenty), 
and thus for periods which are much longer than those 
we have estimated as necessary for their transference to 
our planet. 

On the road from the earth the germs would for about 
a month be exposed to the powerful light of the sun, 
and it has been demonstrated that the most highly re- 
frangible rays of the sun can kill bacteria and their spores 
in relatively short periods. As a rule, however, these 
experiments have been conducted in such a manner that 
the spores could germinate on the moist surface on which 
they were deposited (for instance, in Marshall Ward's 
experiments). That, however, does not at all conform 
to the conditions prevailing in planetary space. For 
Roux has shown that anthrax spores, which are readily 
killed by light when the air has access, remain alive when 
the air is excluded. Some spores do not suffer from in- 
sulation at all. That applies, for instance, according to 
Duclaux, to Thyrothrix scaber, which occurs in milk and 
which may live for a full month under the intense light 
of the sun. All the botanists that I have been able to 
consult are of the opinion that we can by no means as- 



sert with certainty that spores would be killed by the 
light rays in wandering through infinite space. 

It may further be argued that the spores, in their 
journey through universal space, would be exposed dur- 
ing most of that period to an extreme cold which possibly 
they might not be able to endure. When the spores 
have passed the orbit of Neptune, their temperature will 
have sunk to 220, and farther out it will sink still 
lower. In recent years experiments have been made in 
the Jenner Institute, in London, with spores of bacteria 
which were kept for twenty hours at a temperature of 

252 in liquid hydrogen. Their germinating power was 
not destroyed thereby. 

Professor Macfadyen has, indeed, gone still further. 
He has demonstrated that micro-organisms may be kept 
in liquid air (at 200) for six months without being 
deprived of their germinating power. According to what 
I was told on the occasion of my last visit to London, 
further experiments, continued for still longer periods, 
have only confirmed this observation. 

There is nothing improbable in the idea that the ger- 
minating power should be preserved at lower tempera- 
tures for longer periods than at our ordinary tempera- 
tures. The loss of germinating power is no doubt due 
to some chemical process, and all chemical processes 
proceed at slower rates at lower temperatures than they 
do at higher. The vital functions are intensified in the 
ratio of 1 : 2.5 when the temperature is raised by 10 C. 
(18 F.). By the time that the spores reached the orbit 
of Neptune and their temperature had been lowered to 

220, their vital energy would, according to this ratio, 
react with one thousand millions less intensity than at 10. 
The germinating power of the spores would hence, at 

- 220, during the period of three million years, not be 



diminished to any greater degree than during one day at 
10. It is, therefore, not at all unreasonable to assert that 
the intense cold of space will act like a most effective 
preservative upon the seeds, and that they will in conse- 
quence be able to endure much longer journeys than we 
could assume if we judged from their behavior at or- 
dinary temperatures. 

It is similar with the drying effect which may be so in- 
jurious to plant life. In interplanetary space, which is 
devoid of atmosphere, absolute dryness prevails. An 
investigation by B. Schrober demonstrates that the green 
alga Pleurococcus vulgaris, which is so common on the 
trunks of trees, can be kept in absolute dryness (over 
concentrated sulphuric acid in a desiccator) for twenty 
weeks without being killed. Seeds and spores may last 
still longer in a dry atmosphere. 

Now, the tension of water vapor decreases in nearly 
the same ratio as the speed of the reaction with lower 
temperatures. The evaporation of water i. e., the dry- 
ing effect may hence, at a temperature of 220, not 
proceed further in three million years than it will in 
one day at 10. We have thus several plausible reasons 
for concluding that spores which oppose an effective re- 
sistance to drying may well be carried from one planet 
to another and from one planetary system to another 
without sacrificing their vital energy. 

The destructive effect of light is, according to the ex- 
periments of Roux, no doubt due to the fact that the 
rays of light call forth an oxidation by the intermediation 
of the surrounding air. This possibility is excluded in 
interplanetary space. Moreover, the radiation of the sun 
is nine hundred times fainter in the orbit of Neptune 
than in the orbit of the earth, and half-way to the near- 
est fixed star, Alpha Centauri, twenty million times 



feebler. Light, therefore, will not do much harm to the 
spores during their transference. 

If, therefore, spores of the most minute organisms 
could escape from the earth, they might travel in all 
directions, and the whole universe might, so to say, be 
sown with them. But now comes the question: how 
can they escape from the earth against the effect of 
gravitation? Corpuscles of such small weight would 
naturally be carried away by any aerial current. A small 
rain -drop, ^V mm. in diameter, falls, at ordinary air 
pressure, about 4 cm. per second. We can calculate from 
this observation that a bacteria spore 0.00016 mm. in 
diameter would only fall 83 m. in the course of a year. 
It is obvious that particles of this minuteness would be 
swept away by every air current they met until they 
reached the most diluted air of the highest strata. An 
air current of a velocity of 2 m. per second would take 
them to a height where the air pressure is only 0.001 
mm. i.e., to a height of about 100 km. (60 miles). But 
the air currents can never push the particle outside of 
our atmosphere. 

In order to raise the spores to still higher levels we 
must have recourse to other forces, and we know that 
electrical forces can help us out of almost any difficulty. 
At heights of 100 km. the phenomena of the radiating 
aurora take place. We believe that the aurorse are 
produced by the discharge of large quantities of nega- 
tively charged dust coming from the sun. If, therefore, 
the spore in question should take up negative electricity 
from the solar dust during an electric discharge, it may 
be driven out into the sea of ether by the repulsive charges 
of the other particles. 

-We suppose, now, that the electrical charges like 
matter cannot be subdivided without limit. We must 



finally come to a minimum charge, and this charge has 
been calculated at about 3.5.10"" 10 electrostatic unit. 

We can, without difficulty, calculate the intensity of 
an electric field capable of urging the charged spore of 
0.00016 mm. upward against the force of gravity. The 
required field-strength is only 200 volts per metre. Such 
fields are often observed on the surface of the earth with 
a clear sky, and they are, indeed, almost normal. The 
electric field of a region in which an auroral display takes 
place is probably much more intense, and would, without 
doubt, be of sufficient intensity to urge the small elec- 
trically charged spores which convection currents had 
carried up to these strata, farther out into space against 
the force of gravity. 

It is thus probable that germs of the lowest organisms 
known to us are continually being carried away from 
the earth and the other planets upon which they exist. 
As seeds in general, so most of these spores, thus carried 
away, will no doubt meet death in the cold infinite 
space of the universe. Yet a small number of spores 
will fall on some other world, and may there be able to 
spread life if the conditions be suitable. In many cases 
conditions will not be suitable. Occasionally, however, 
the spores will fall on favorable soil. It may take one 
million or several millions of years from the age at which 
a planet could possibly begin to sustain life to the time 
when the first seed falls upon it and germinates, and 
when organic life is thus originated. This period is of 
little significance in comparison with the time during 
which life will afterwards flourish on the planet.' 

The germs which in this way escape from the planets 
on which their ancestors had found abode, may either 
wander unobstructed through space, or they may, as 
we have indicated, reach outer planet?, or planets moving 



about other suns, or they may meet with larger particles 
of dust rushing towards the sun. 

We have spoken of the Zodiacal Light and that part of 
it which has been designated the counter-glow. This 
latter glow is regularly seen in the tropics and occasion- 
ally in that portion of our heavens which is just oppo- 
site the sun. Astronomers ascribe the counter-glow to 
streams of fine dust which are drawn towards the sun 
(compare page 147). Let us assume that a seed of the 
diameter of 0.00016 mm. strikes against a grain of dust 
which is a thousand times as large (0.0016 mm. diameter), 
and attaches itself to its surface. This spore will be car- 
ried by the grain of dust towards the sun; it will cross 
the orbits of the inner planets, and it may descend in 
their atmospheres. Those grains of dust do not, by any 
means, require very long spaces of time to pass from 
one planetary orbit to another. If we assume that the 
spore starts with zero velocity near Neptune (in which 
case the seed might originate from the moon of Neptune; 
for Neptune itself, like Uranus, Saturn, and Jupiter, 
is not yet sufficiently cooled to sustain life), the spore 
would reach the orbit of Uranus in twenty-one years, and 
of Mercury in twenty-nine years. With the same initial 
velocity such particles would be twelve years in passing 
between the orbits of Uranus and Saturn, four years be- 
tween Saturn and Jupiter, two years between Jupiter and 
Mars, eighty-four days between Mars and the earth, forty 
days between the earth and Venus, and twenty-eight days 
between Venus and Mercury. 

We see from these time estimates that the germs, to- 
gether with the grains of dust to which they have at- 
tached themselves, might move towards the sun with 
much smaller velocity (from ten to twenty times smaller) 
without our having to fear any loss of their germinating 



powers during the transit. In other words, if these seeds 
adhere to the particles, ninety or ninety -five per cent, of 
whose weight is balanced by the radiation pressure, they 
may soon fall into the atmosphere of some inner planet 
with the moderate velocity of a few kilometres per second. 
It is easy to calculate that if such a particle should, in 
falling, be arrested in its motion after the first second, it 
would yet, thanks to the strong heat radiation from it, not 
be heated by more than 100 Cent. (212 F.) above the 
temperature of its surroundings. Such a temperature 
can be borne by the spores of bacteria without fatal 
effects for much more than one second. After the par- 
ticles, together with the seed adhering to them, have 
once been stopped, they will slowly descend, or will be 
carried down to the surface of the nearest planet by de- 
scending convection currents. 

In this way life would be transferred from one point of 
a planetary system, on which it had taken root, to other 
locations in the same planetary system which favor the 
development of life. 

The seeds not caught by such particles of dust may 
be taken over to other solar systems, and finally be 
stopped by the radiation pressure of their suns. They 
cannot penetrate any farther than to spots at which the 
radiation pressure is as strong as at their starting-points. 
Consequently, germs from the earth, which is five times 
as near the sun as Jupiter, could approach another sun 
within a fifth of the distance at which germs from Jupiter 
would be stopped. 

' Somewhere near the suns, where the seeds are arrested 
by the radiation pressure to be turned back into space, 
there will evidently be accumulations of these seeds. The 
planets which circulate around their suns have therefore 
more chance of meeting them than if they were not in 



the vicinity of a sun. The germs will have lost the great 
velocity with which they wandered from one solar system 
to another, and they will not be heated so greatly in fall- 
ing through the atmospheres of the planets which they 

The seeds which are turned back into space when 
coming near a sun will there perhaps meet with particles 
whose weight is somewhat greater than the repelling 
power of the radiation pressure. They would, therefore, 
turn back to the suns. Like the germs, and for similar 
reasons, these particles would consequently be concen- 
trated about the sun. The small seeds have, therefore, 
a comparatively better chance of being arrested before 
their return to space by contact with such particles, 
and of being carried to the planets near that sun. 

In this manner life may have been transplanted for 
eternal ages from solar system to solar system and from 
planet to planet of the same system. But as among the 
billions of grains of pollen which the wind carries away 
from a large tree a fir-tree, for instance only one may 
on an average give birth to a new tree, thus of the 
billions, or perhaps trillions, of germs which the radiation 
pressure drives out into space, only one may really bring 
life to a foreign planet on which life had not yet arisen, 
and become the originator of living beings on that planet. 

Finally, we perceive that, according to this version of 
the theory of panspermia, all organic beings in the whole 
universe should be related to one another, and should 
consist of cells which are built up of carbon, hydrogen, 
oxygen, and nitrogen. The imagined existence of living 
beings in other worlds in whose constitution carbon is 
supposed to be replaced by silicon or titanium must be 
relegated to the realm of improbability. Life on other 
inhabited planets has probably developed along lines 



which are closely related to those of our earth, and this 
implies the conclusion that life must always recommence 
from its very lowest type, just as every individual, how- 
ever highly developed it may be, has by itself passed 
through all the stages of evolution from the single cell 

All these conclusions are in beautiful harmony with 
the general properties characteristic of life on our earth. 
It cannot be denied that this interpretation of the theory 
of panspermia is distinguished by perfect consistency, 
which is the most important criterion of the probability 
of a cosmogonical theory. 

There is little probability, though, of our ever being 
able to demonstrate the correctness of this view by an 
examination of seeds falling down upon our earth. For 
the number of germs which reach us from other worlds 
will be extremely limited not more, perhaps, than a 
few within a year all over the earth's surface ; and those, 
moreover, will presumably strongly resemble the single- 
cell spores with which the winds play in our atmosphere. 
It would be difficult, if not impossible, to prove the 
celestial origin of any such germs if they should be found 
contrary to our assumption. 






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